troubleshooting wifi networks

Restrict your APs to use channels 1, 6, and 11. If they can use them all they hop around the place and often end up in the wrong place.

Check controller, check uptime.
Check controller and AP uplinks
Are WLANs properly segregated ?

Download and install Inssider and review other networks. Around. Watch during the time of the issue are other networks appearing ?

Signal strength (db)
closer to 0 is better
acceptable range
-30 to -90
I get -40 when right beside the AP
-30 to -50 = good
-60 to -70 = decent
you want at east -75
-70 to -90 will work but performance will be bad

Are clients 2.4G or 5G ? Is there lots of networks on 2 but 5 is free ?
Are all devices compatible

If you are still having issues. You can look at other radio waves interfering but need more hardware (see inssider and wispy)