I have seen ubuntu automatically set the bit rate to the lowest value by default.
type:
iwconfig <adaptername>
e.g. ath0, eth1, rausb0, wlan0..
This should tell you your Bit Rate, make sure it's right.
you can change it if it is low, say you want 11Mbps or 54Mbps, do:
iwconfig <adaptername> rate 11M
or something like that.