check if any interfernce on 2.4 GHz channels. Did you do the site survey porperly ?
Try to chnage the channels on 2.4 GHz!!!!
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Also, you could try to disable lower data rates (802.11 b rates) from the WLC if not needed. This can be done from: Wireless -> 802.11b/g/n -> Network -> 1 - 11 Mbps disabled, 12 Mbps Mandatory and the above rates Supported.
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