Winterhoff hitch - stuck

Flutel

I've just taken delivery of a new van with a Winterhoff hitch (I used to have an Alko).  When  uncoupling it, at my storage, it was quite tough to do.  Then, when I tried to put the 'operating lever' down, it stuck fast in the up postion!  Can anyone throw
any light on this before I have to get someone else to look at it?