I have a '98 Boxster and have replaced the door latch mechanisms and ignition assembly in my car roughly 1 year ago.  Everything has been working as expected during this time up until last Sunday.  The key works in the ignition and I can unlock the car using the key in the door, but I can no longer lock the car by inserting the key in the door lock.  I don't have a working fob but I can lock and unlock the car using the button in the dash.  I can also lock the car using the button in the dash and unlock the car using the key in the door - but I can't lock the car using the key in the door.
When I attempt to lock the car using the door lock, the key simply won't turn clockwise.  It feels like it is hitting something.  I have...
- Cleaned the grooves in the key
 
- Put some graphite in the door lock
 
- Opened the door panel to see if there is something physically preventing the lock from spinning - didn't see anything
 
Windows move as expected when you open/close doors, etc.  Anyone run into this?  Or have any recommendations?  As much fun as it was installing the door latch mechanism, I would prefer not to do that again if I don't have to.  Is there a position in the door latch mechanism that can prevent the key from moving in the lock position (clockwise motion)?