I had the very same problem, pick was always spinning around / sliding away from me. I watched (and rewatched) the beginner pick video here and scoured youtube videos to find some clue to what I was missing. I don't know if it was chance, or if something clicked from it, but this is the video I ran across that seemed to make the fix for me.

https://www.youtube.com/watch?v=1JBxNFEg0nU

I started off using the yellow Tortex which is when I was having the slipping issue. I switched to the .38 white nylons he shows and its thinness (being more forgiving than the thicker picks) allowed me to focus more on form while strumming instead of constantly dealing with the pick moving around.

After a while, I switched back to the yellow Tortex and found that the issue was...well...much less of an issue. I was even able to just pick up a Jazz III pick shortly after and use it for clarity working on scales and such. Hope this helps!

(As a point of reference, I just started learning guitar in January.)