I would check the "show client detail <client_mac_add> " output & verify client connectivity details. See below thread as I have posted what you should looking for.
https://supportforums.cisco.com/discussion/12710671/not-able-achieve-80211n-bandwidth
If low data rates (anything below 12Mbps) enabled in 802.11a or 802.11b/g band , I would disable them as well.
Most of the time these low throughput depend on client capability as well. so check client specs as well.
HTH
Rasika
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