Something's not right there. Despite it being much more common to push the button down, you should still be able to lock the door with the key. Having said that, the passenger door on my car is much stiffer to lock with the key than the driver's side, but I've always just put that down to the comparative lack of use. When you've got bucket seats and a floor-shift you don't tend to slide across from the passenger's side too often
Can you take the barrel out and operate the lock mechanism with a screw-driver? That will at least tell you where the problem is.
hth,
RET