Push button lock unlocks when knob turned
It sounds to me like something in the knob has worn out (eg, a metal locking mechanism has broken off or been worn down). If this is the case, there's not much chance that you'll be able to [easily] fix it. 

If it was me, I'd just replace the knob. You may also be able to swap it with another knob in a location where you don't need a working lock.