Check as well what the encryption the ssid is using. If it is set to TKIP then as per the standard, that will only use legacy rates and not the HT rates.
I have seen many instances on deployments put in a long time ago that were setup originally with TKIP and even though they have deployed new AP105s, they are stuck on legacy rates.
But, yes as cjoseph said, you should open a case with TAC if this is a recent occurance.