First check the router's wireless mode matches the adapter's -- mixed mode is a safe bet. Also check that Access List (filter by MAC ID) is disabled.
You should also try optimising reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.