I had the same no apparent leak....however, I think that my leak was when the gas was attached to the post.....it let gas in to a certain extent and then it slowly escaped. I was able to carbonate it, so I think my issue was when I unhooked and rehooked it up at some later time.
Bingo! Swapped the c02 from the other keg and it did the trick. Seems like the one adapter is allowing gas into the keg, but like you say, it was either leaking out there or allowing only a small amout into the keg. Anyway, I rolled it on the floor for a cpl minutes, checked it this morning and its carbonated.
Thanks for your help, gentlemen