Theymans,
there could be a number of reasons, scan rejection most likely, be it due to load, scanning disabled in ARM, voip aware being held up, rogue containment or some acl with 'disable scan' being hit etc.
Check the output of
show ap arm rf-scan-times ap-name <the_ap>
specifically the section on "WIF Channel Scanning State" will give you an idea of the reject ratio:
scans (Tot:Rej:Eff(%):Last intvl(%))
180365:12733:92:100
1238085:0:100:100
what do you see happening ? is reject increasing ? whats the efficiency ?
also look at "show ap debug radio-stats ap-name <ap> radio 1 advanced | include Scan" a few times - this may give a clue where the problem is, i.e. which reject is incrementing frequently
Here is an example where I know there is video running over this AP, it's exhibiting a similar symptom to your case, but I am quite OK why it's happening.
(sg-7030) #show ap arm rf-summary ap-name ap92
Channel Summary
---------------
channel retry phy-err mac-err noise util(Qual) cov-idx(Total) intf_idx(Total)
------- ----- ------- ------- ----- ---------- -------------- ---------------
1 0 0 0 0 0/0/0/0/0 0/0(0) 0/0//0/0(0)
5 0 0 0 93 16/4/2/0/90 0/0(0) 0/0//0/0(0)
6 0 0 0 93 25/22/0/0/89 0/0(0) 0/0//0/0(0)
7 0 0 0 0 0/0/0/0/0 0/0(0) 0/0//0/0(0)
11 0 0 0 0 0/0/0/0/0 0/0(0) 0/0//0/0(0)
(sg-7030) #show ap debug radio-stats ap-name ap92 radio 0 advanced | include Scan
Scan Requests 2053
Scan Rejects 2036
Load aware Scan Rejects 0
PS aware Scan Rejects 0
EAP Scan Rejects 0
Voice aware Scan Rejects 0
Video aware Scan Rejects 2036
UAPSD Scan Rejects 0
CABQ traffic Scan Rejects 0
Radio Reset Scan Rejects 0
Queue Drain Scan Rejects 0
Scan Success 17
Rx ARM Scan Frames 146
regards
-jeff