Hi Inge,
I had trouble with my pick spinning around, not with dropping it (not very often). It took some time paying mental attention to my pick position and what made it spin to get things to work well.
I think your mojo grip has been a crutch that did not serve your learning well. I did the same using the app to play songs without learning them. I finally made the decision to just learn and drop the crutch. It was a good decision.
Take a look at my general answer to strumming angles. My guess is that you are approaching the string with an angle on the pick that lets it dig in. If you wanted to make a video that will help the guessing it would improve the accuracy of the answers here. A photo is ok, but you need to make sure it shows what your true approach is, not what you think it is. 