AL-KO Jockey Wheel failed

I have a 2018 van and the handle on the jockey wheel has failed. The handle is held on to the central barrel of the jockey wheel by two small grub screws. After hitching up, I raised the jockey wheel and tightened up the handle. Bang, the screws came out and the central barrel fell to the floor. I was able tie up the wheel until I could look at it properly. I have since taken the grub screw barrel (in the central barrel) and replaced the whole lot with an M4 bolt. Job done I thought, but on a different trip out, tightening up the handle of the jockey wheel once hitched up, the M4 bolt sheared completely and again the jockey wheel fell to the floor. Fortunately on that occasion the angle of the can and van, I was able to take the whole jockey wheel out and just tow without it until I could fix it.

I have been towing for 10 years. This is my second van. Yes, I could be tightening up the jockey wheel handle (for fear of it winding down in transit) too much, but really? I would be very surprised I could do that. And put in enough force to shear an M4 bolt? 

So Question: Am I doing something wrong, or is this a problem others have found? 

thanks in advance