Will hidden nodes issue cause high re-transmission on both AP and STAs ?
You don't say whether this was on one device or many. Might you have cell phones operating in dark (RF wise) corners of the building ?
Have you tried enabling RTS/CTS ? This is NOT a good (permanent) fix, but is useful for diagnosing the problem. If you turn on RTS/CTS, on the suspected device, and throughput increases you have confirmed the problem. Try setting the threshold length to about 400 bytes, or even lower in a warehouse .
Packet captures can help diagnose this too.
i also have problem with RTS.
Have you tried enabling CTS-to-Self instead of RTS/CTS ?