Westfalia tow bar fitting issue

[Horrgakx]

Here are 2 photos and 1 video ...

From the video you'll see it will insert but not lock.
From the photos you'll see the knob doesn't drop into the locked position.
Initially it looked like the knob was getting caught on the bumper trim, it's tight, but there's a millimetre or so gap so it will be able to rotate.

https://youtu.be/kqEewI3ETEA

[The_Laird]

The housing and the tow hook can both get pretty corroded. I’ve used a flap wheel on a drill to remove most of it from the housing, and loads of WD40 and grease (you can see the grease in at least one of my photos. I also tried using a jack to gently push the tow hook into place.
Initially, it took ages to get it to free up, so a combination of patience and a bit of force might be required!