Actually, after re-reading your post it seems that it would be the second case. If that's so, it's likely that:
1- the lockbar is binding between the liners at the blade tang or-
2- the lockbar is binding in the back where you press or-
3- the blade tang is binding between the liners when closed
A few things to check- first oil it good and see if that helps, then open the knife, and while holding the blade open (all the way), work the lock. Does it move quite freely or is it very sluggish?
- now open the knife about halfway, and while pressing down on the lockbar, open the blade the rest of the way. Does the blade go to the full open position easily with nothing grabbing it at the last moments?
- Now with the knife fully open, close the blade about halfway, then depress the lockbar and let the blade close (you're going to have to finesse your hand position a bit to keep you thumb out of the way!). Does the blade drop in freely without getting caught?