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Recently, while I was sniffering the packets, I found some of clients would send the ADDBA action frame during the 4-key handshake phase, and some of clients would fail to connect with the AP due to the EAPoL frame retransmission, but the channel busy is fine, not very high, below 50%. 

thanks in advance.

qq

Clients -- 80% of WiFI issues are generally clients for various reason. Bad drivers, mic config, buggy, poor roamers etc etc etc ..

Without specifics its hard to say -- 

ADDBA action frame during 4 way handshake isn't something Ive seen to be honest. Can you share the capture ?

EAPoL frame retries - Know that not all retires are related to noise, while it is a significant contributor most of the time. When you look at retires and if noise is not a  contributor  then you look at things like client configs or if all the clients exhibit the same issue then you look at the AP side of things. Multiple path, if legacy protocols are in use, can cause retires while still having low channel busy. 

I agree.  Most likely a Client error. 

Its state machine (in the driver) is poorly designed/written.

It could be that a previous wireless connection has been broken, and the client doesn't realize it.  Perhaps a timeout should have occurred, but was ignored.