I agree with what Weenie, Todd, Eagle, and Jbug have all suggested.
Constant gas pressure, connected and flowing all the time, at 13 psi will EASILY carbonate a corny keg in a week (though the bubbles and head seem tighter and just overall better after 2 weeks IMHO).
If that is happening, then either you have a mechanical problem where the pin cannot push the poppet in to let the gas flow into the keg, or the pressure readings are low and so you are in fact carbonating but it is at such a low pressure that it still seems flat.
This zymurgy article is a SUPERB one on everything kegging related.
http://www.homebrewersassociation.org/a ... How-To.pdf