I removed with a bit of trouble the headrests from the second row of seats.
The problem is that I cannot get back in one of them. The darn thing will not budge a half inch (I am pressing the plastic post by the seat). The other went in with no problem.
Any ideas?

 

do you mean the post or do you mean a button ?

 

The plastic button by the seat.

 

Sounds like a jammed button. Did you try switching the head rest from the good side to the bad side to see if there is an issue with the headrest itself or the rods ? That may help isolate the problem to the button if both head rests have problems going in just that one side. Is the car still under warranty ? If so and if it is a jammed headrest button, hopefully that is a free fix at the dealer.