Yes earlier i responded for specific message you were getting, there is possibility user unable to connect due to high noise, you can download free wifi scan app and see the noise in the environment, your AP is on fixed channel or it can change chann...
yes you can extend coverage to next building using MESH solution, now how you implement MESH is totally dependent on clutter / terrain / LOS / NLOS. You can either use back to back direction AP solution, use eMESH between those 2 APs and then MESH it...
All the clients getting rejected or particular client is getting rejected? Basically there is feature in controllers to watch for spoofing, so for example if 2 DHCP request coming from same MAC address it will be detected and controller will take act...