I have a Whitter detachable towbar that I only use with my rear mounted bike carrier.

When I attempt to attach the ball it to the socket on the car the blue knob doesn’t snap all the way round and allow me to lock it without several attempts and significant amounts of swearing/grazed knuckles.

I’ve tried lubricating the mechanism of the towball but this doesn’t seem to have helped.

Can anyone suggest how to free up the lock mechanism?

It seems to work OK when its off the car.

The only issue I can think of is there is a silver pin that should be protruding ( I think) when you remove the tow bar. But as the mechanism is quite stiff sometimes when you remove the tow bar you don’t fully rotate the knob and lock the mechanism so when you remove the tow bar the spring loaded silver pin retracts back into the tow bar. When this happens when you come to refit the tow bar the mechanism doesn’t engage and lock into place.

As above.. you didnt read the manual, did you?