what version of code are your running on the WLC? The ability to ping the AP-manager interface, isn't always trustworthy.
If you can ping the management interface, the AP should be able to reach it as well, and if the AP-manager is in the same subnet, best practice, they should be able to join.
Can you run and capture
debug mac-address < ap mac >
debug capwap events enable
debug capwap errors enable
Steve
HTH,
Steve
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