The previous advice to give it more welly sounds about right -- to get mine to insert the last inch requires quite a good whack with the flat of my hand (in best Will Smith style....). Phil at Allon White can probably give you a more technical explanation. Not sure where those flattened tubes came from originally, but I had the exact same headrest setup in all of my TVR M-Series and S-Series, so I'm guessing they have been around for at least 4 decades. (The TVR's had a different method of finishing the leather seatbacks, which meant it was easy to get your hand "up there" to figure where all the bits fitted).
![[Linked Image]](https://www.tm-img.com/images/2022/08/02/TVR-S-Series-headrests.jpg)