Sometimes with your kegs the seal on your lids can get a bit weak.  It basically feels like the legs of the lid aren't pushing down hard enough.  There are two easy fixes to this.  First, you can put some extra rubber or something a few millimeters thick underneath the feet to push it up a bit.  With a little adjustment you can usually get something pretty good with that.

Alternately, if you have a lid that has metal loops which secure the locking arm, you can take a hammer and beat down the top of the loops a bit to reduce the vertical distance that the arms have to swing.  (Not all kegs have these kind of loops so please don't try this with other types).  It doesn't take too much effort so give it a few hits, see if it has improved, and adjust as required.  Before you hammer on it make sure you have it on a flat surface, preferably with a small amount of padding under the lid to avoid denting it on something below.  See the picture for the proper areas to hit.