jit17,
How do you know this is only occurring when scanners are associated to just the 1 AP? Have you verified this?
Do scanners NOT have this issue when associated with any of the other 3 APs?
If you have verified that this is only happening on the 1 AP:
My assumption would be that the config on all 4 APs would be the same (except power/channel, hostname, IP address, etc). If that is also true, then it points to an issue with the RF in the area near that AP.
Are the scanners using 2.4 or 5GHz WLANs? (or both?)
If using 2.4GHz, as a test, try to setup the same WLAN on the 5GHz radio on just that 1 AP and see if the problem continues.