Make sure your fgt device is running the special " wireless controller" build - as far as I am aware full support for the 221C hasn' t been integrated into the main steam firmware builds. (You can download the special build under the sub-directory under the FortiAP folder/verson/...). Read the firmware patch notes for both the AP and fgt device to confirm they are both compatible with each other and also to see any known issues.
Personally, I would try to upgrade the firmware on the AP to the latest that is supported by the main fgt (or upgrade the firmware on both if needed).
Perform a site or rouge survey to confirm the 221C is not using the same channel(s) as another AP.
Not in front of a fgt at the moment, so not sure of the exact settings, but there is an " hand-off" setting you may want to look into.
Try creating a custom WAP(?) profile for the 221C.
The fgt has a nice set of debug commands for wireless troubleshooting -- may be someone can post the list (currently not at work to check my notes).
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C