While community forum is great for discussions and sharing and it is not as great as straight tech support. To figure out what is the exact reason of the issue you are having I would strongly recommend to open a support ticket, share tech support logs and work with the team. It is available as a free service for all Cambium customers.
PS My feeling that the reason is an interference on the client side. But as you can imagine it is just a guess.
If it was me, I would upgrade to 4.6.1-RC27, which is a mature Release Candidate and as far as I know, it addresses all known/reported problems. We are running it on a bunch of ePMP APs and SMs, and it’s working well for us.
No Allocation on AP can mean one of two things: 1) the AP is full as either you have max number of SMs for that AP or you have an AP-Lite; OR 2) your SM has been banned from that AP by the MAC filter.
I would start like Ninedd said and upgrade to 4.6.1-RC27 on both that AP and SM, while your in there make sure your max subscriber count in the config is larger than the number of SMs on the sector. Also, check the MAC filter and make sure that SM isnt in the list and boot SMs off that sector is they arent supposed to be on that one, the mac filter list is a good way to prevent SMs from migrating where they are not supposed to.