What kind of valve, Presta or Schrader? In a presta valve you need to unscrew the top a bit in order for air to enter.
How many PSI are you trying to fill to? Does your compressor go that high? Some of those smaller electric compressors only do 30PSI and many bike tires will require 100+ PSI.
Also, we have a cycling thread here where you can post questions like this, come join us:
http://forum.calgarypuck.com/showthread.php?t=106340