With hub in pairing mode, until the device pairs, continue pressing the small button found on the perimeter edge of these buttons every few seconds. If there is no button on the edge but a small hole on the back, same deal, using a paper clip depress the switch in that small hole every few seconds.
You should see an led light blink several times as you press that button or switch. I've found these buttons and temp sensors pair fairly easy using this method. And if they ever drop off your mesh you can press these items the same way and they will rejoin even if the hub is not in pairing mode.
I'm using @veeceeoh's device handlers for my Xiaomi devices and have switched to Ikea Tradfri smart plugs for repeaters.
