Hi Herby. Do you think that the tow hitch needs to move forward a little more to expose more of the slot? If so, then it's a similar problem to that which some of us get when trying to unhitch the van. By applying the handbrake and then driving/pushing the tow car forward by a few mm's, the tow hitch is then at full stretch.
Perhaps that's what you need to do?
Also, it wouldn't harm to pump a little grease into the mechanism using the grease nipples shown on your photo.
Randa