Pressed the reset until if flashes 3 times.
Done this numerous times.
Removed battery for a few minutes.
Replaced the battery.
Rebooted C7.
Shut down C7 and removed power for a minute.
Tried it at various distances from the C7. Including on top of it.
Two of these were supposed to turn up but only one did (kind of glad about that!).
Anyone have any ideas before I sling it and buy ST ones (I have 7 of these and they all paired within a couple of seconds).
How many total battery zigbee devices do you have connected to your hub? It's possible you've hit the limit and need more repeaters.
That said, I have a few Sonoff devices, and they are difficult to pair. They seem to go to sleep pretty quickly after you initiate pairing, so if the hub doesn't find them quickly, it won't find them at all.
Thanks. This is the 8th one.
I have 5 ST Outlets/Plugs too.
Other devices (10 or so) are wi-fi and z-wave,
I wil give it a few more goes later. Maybe it is sleeping quickly, like you say, but I do see it flash red a couple times during the first 15 seconds of the process, about 7 seconds apart, (after the intial 3 flashes).
Honestly, I don't think I'm going to buy any more of the Sonoff motion sensors. The low cost isn't worth putting up with the shaky performance. You mentioned using the Samsung sensors, which were a great alternative. Sadly though, it looks like Samsung is getting out of the hardware business. In Europe, Aeotec is now distributing the Samsung sensors, and charging Aeotec prices for them. So far, there doesn't seem to be anyone distributing them in the States. I loved those sensors, but not if they cost more than $40.
Treid another 10 times. Still the same.
Told amazon only one in the box and that was faulty so not to send the missing one.
They have also jacked the price up today from £13.99 each (which I thought was expensive compared to other places), to £21.62.
So I will give them a miss too.
These were to go outide (though sheltered from rain). Bit wary to spend more on an ST one (if I could find one) and put that outside. Liked the size of the sonoff too. Ah well, will see what else is available.
I would look at the Hue outdoor sensor for that.
Thanks, that might be an option. Was trying to save costs and thats a bit larger (and uglier) than I wanted.
I do have a Reolink camera (in Synology Surveillance Station) thats pointed in the same area I need to cover, so might see if/how that works. Not PIR and does give a fair few false alerts in the wrong weather conditions though.
If it does not work out I will take another look at the Hue 
Edit:
Actually if the camera motion sensor works this will only be temporary as I want the temperature sensing too (edit: which I now realise these motion sensors do not have).
Strange. I'm gradually moving to all Sonoff sensors.
Just for info, I'm sure you've tried this, but this is what I do.
Put hub in zigbee discovery mode.
Hold button in till the led flashes 3 times.
Release button.
9 times out of ten the hub discovers the device.
When the hub finds the device, sometimes I will press the button a couple of times to keep it awake, but usually it will then find the handler for it and I'm good to go.
You are probably doing this but just thought I'd mention it.
How many total Zigbee devices do you have, and how many are repeaters? Without repeaters you're limited to 32 Zigbee devices. Since it sounds like you're adding a bunch at one time, that's one consideration that comes to mind (besides whether you have any possibly bad repeaters like smart bulbs).
Thanks Bobbles. Yes, that is exactly the process I have been following.
Probably tried at least 30+ times now.
I reckon I have faulty device.
Will give it another go in the morning, before returning it.
Only repeaters are the 5 ST Outlets/Plugs. Just 13 zigbee devices in total and all others added a week ago when I got the hub.
I just remebered I have an Echo Plus.
Just tested with that and it found it straight away (I noticed the LED glowing bright red so knew instantly it was different).
It has it as: First motion sensor eWeLink smart device.
The Alexa app shows it but states: "there is a problem".
So not exactly faulty.
Try pairing with the ST plugs unplugged. The sonoffs may not like them.
I just tried that. Still not discovered.
Well! Amazon sent me another one.
This is also not discoverd.
I had a spare and previously unused outlet/plug and this was detected in a few seconds so there is seemingly nothing wrong with zigbee on my hub.
Is there any way to turn on some sort of advanced logging to see what happens with the discovery process.
If not, I hate to do it, but I am going to have to reinstate my ST hub for these devices, and see if they work on that. 
How do I complety reset the C7 hub? Do I have to get support involved to reregister it?
I am guessing I can save my rules and devices and add them back in one by one?
I appreciate you are having issues but what do you think a complete reset of the hub will do?
What zigbee channel are you using?
I'm not having any issues pairing on keeping these devices online. My zigbee channel is 20.
I'm not sure if it is the same for Sonoff devices, but for Xiaomi/Aqara their proprietary hubs use 15, 20, 25. (Not 100% sure on this but just something I think I may have read).
Also what channel is your router using for wifi?.
If the channels overlap this can cause issues.
Just some thinking out loud.
Zigbee 20. Wifi 6 so should be no overlap?
Though with the first one I tried disabling the 2.4g on the router.
I have some wifi lights could they interfere?
Why resetting the hub? To see if a device/s or a setting is messed up somewhere and causing this.
Others use these devices so there must be something local to me.
Sounds like you have things covered.
In that case I would try a soft reset first.
Do a backup of your database, perform a soft reset and then re-install your backup.
I would try this before the more drastic step of a full reset.
Just tried again with all wifi lights off and 2.4g off. Still no good.
So the soft reset.
Try and add the new sensor straight afterwards before the database restore?
If it gets added the database restore will remove it again?
Download the Hubitat app
