Before you slide the headlamp in and lock it into place? I always struggle with this as it seems to be more of trial and error than anything else.

 

According to the manual, you half turn to unlock then quarter turn BACK. Then you slide it in and do the other quarter turn.

What this means is the the 'u' should be at the bottom before you slide it back in.

 

I kept the locking mechanism about where it was when it first unlocked (clicked) and the lamp unit seemed to slide back in easily. I recall needing to make sure I lined the unit up properly and straight before sliding it back into the housing.