It's possible.....if the the o-ring is bad for example. Dont' forget to use a little keg lube on the o-rings and posts once in a while too. If it is leaking while connected you might be able to hear it if you get your ear close enough. Then again maybe not. One way to find out for sure would be to shut off the valve to the already carbed keg. Leave the valve open for the second keg and then shut off the tank. Wait a few hours and go back and check the gauge. The guage should still read pretty close to where it was when you shut off the tank. If it has dropped substantially, you probably have a leak.
If you rule out a leak....it must be the partially closed valve that was causing your problem.