We had similar issues with Apple devices sticking and not being able to connect, low signal, the whole kit.
Coverage is not an issue with this location, plenty of APs
Spoke with TAC and they recommended the following changes.
1. Station handoff Assist set to 25
2. Local Probe Request Threshold (dB) set to 30
3. Deny_Broadcast Probes (unchecked)
4. Opportunistic Key Caching (uncheck it)
5. Check certificate common name against AAA server (uncheck it)
I did a bit more research and enabled Validate PMKID.
The changes seemed to help quite a bit but recently, they complained about 2 iPads not being able to connect.
Looking at Airwave and some logs, I posted this
http://community.arubanetworks.com/t5/Wireless-IPS-and-Content/Block-Ack-Attack-the-cause-of-wifi-outage/td-p/48270
Looking at the trap, it does not seem like the Block Ack Attack was the cause but the user's SNR was 10, they were probably trying to connect to an AP too far away.
I then called TAC and they are now recommending to enable DOS Prevention.
I figure also we should remove the G data rates 1 and 2.
I am not too sure what to do at this point