The bandwidth available on the wireless link is largely dependent on the signal quality and the signal strength. If either or both are reduced, so too is the throughput on the link.
Check the client-to-Access Point (AP) link by performing the following options:
The problem might not be a wireless problem. Congestion on the wired network to which the AP is connected can impact end-to-end throughput.
Poor connectivity / Performance
WLAN adapters (wireless card) / ACU (Aironet Client Utility)