This could be due to database error. Running the following SSH command should fix it
service unifi stop
mongod --repair --dbpath /srv/unifi/data/db
service unifi start
How to Establish a Connection Using SSH
https://community.ubnt.com/t5/UniFi-Wireless/Restore-Cloudkey/td-p/1957403
Weak signal, Access points not meshing with each other even though they are within range.
This could be due to weak transmit power by the 5G radio. In order to increase the Transmit Power, follow these steps but keep in mind that this will disconnect the access point for 3-5 minutes. Also keep an eye on the interference to ensure there isn’t a large increase as this may also cause poor performance.
1. Log into the Unifi Cloud Controller for your network
2. Find the Access Point with a weak signal by using the map tool with the 5G layer enabled
3. The Access Point with the weak signal will have a smaller ring
4. Click on the settings icon for the Access Point with the weak signal and choose the Config menu
3. Set Transmit Power to Custom and increase the dBM value. (Recommend 19 dBM)
6. Select Queue Changes, then Apply Changes
Remotely Reset to Factory Settings
After SSH connection is established run the following command:
syswrapper.sh restore-default