I am having dual OS (Windows & Ubuntu) in my Laptop, and a ADSL Modem based WiFi Access Point.
The WiFi Access Point always works fine with Windows, however with Ubuntu sometimes (or infact most of the times) it is not being detected. Further, the Ubuntu always shows other surrounding WiFi networks.
Reason: The possible reason may be conflict of Channel or Frequency with other WiFi networks.
Try to change following settings in your WiFi Access Point:
1. Change Channel from Auto to a fixed channel number. (for example: 07)
2. Change Channel bandwidth to 20Mhz to 20/40Mhz.
3. Restart the Access Point.