How many Zigbee devices do you have, in particular line powered or repeaters?
If you have metal boxes, thick walls (masonry or heavy plaster) air ducts, or other blocking materials, you could have issues.
You also should probably look at what Wifi channel you are on, and what Zigbee channel you are on. There are certain Wifi and Zigbee channels that can interfere with each other. This is a good thread outlining those channels. Zigbee Channel Overlap
