If you have the disconnects attached to the posts then they are depressing the poppets. Point being, the poppets only seal when there is no disconnect attached.
With that said, i had the same problem that i tracked down to being an issue with a poppet. If the gas poppet is stuck or jammed it might not allow the CO2 to enter the keg properly (and the beer won't carbonate).
Here's a site i found really quick with pics for disassembling a corny: http://cruisenews.net/brewing/kegging/index.php
I'd take the posts off, if you have the poppets that can be removed they will just fall out when you push down on them. Inspect for rips on the rubber seal, etc. and replace the entire poppet necessary (rubber seal is part of the valve, it can't be replaced separately). If all is good, soak everything in oxyclean, rinse and reassemble with food grade lubricant.