When that happens, I just put it in a sink full of very hot water and some Dawn dish soap. I tend to forget that a good soaking in hot water tightens the headgear straps up nicely. I'd rather do that than trying to readjust the straps up by hand.
Besides, after several weeks of use, the straps tend to get a little dirty. I give the mask / headgear a good swishing around and some rough hand massaging. When all is said and done, the headgear comes out looking and smelling like new.
Oh yeah.....and my mask doesn't leak anymore. So......if you'e experiencing mask leaks, wash your headgear and see if that helps.