I have a strange one. If I take a new access point to one of our remote and plug it in, it will not join the controllers at the home location (running latest 4.2) but it will eventually find one of our 2106 controllers running 5.1. If I take an access point out and plug it into the network and have it join the controller before it is shipped (but with no configuration changes) it will work just fine. These are new 1130 series access points (factory LWAPP). I had a local tech look at one and he said that it is getting the no join response message, but from the controller debugs I can see the that it is getting a response from the join request. The WAN is a 3mb connection with a an MTU of approximately 1475. The latency is sub 100 MS round trip. The access point is finding the controllers no problem. Clocks are correct on the controller and the 1130's have the MIC so it shouldn't be a problem with that. Main location controllers are 4404's with LAG on the interfaces. One controller has 37 access points connected while the other has 60.
Just to restate. I can take one of the access points that isn't functioning correctly at a remote and if I plug it in to the network at HQ and let it flash up it works just fine at the remote.