I have been playing for several years and the pick turning in my fingers to the flat side of the pick has always been an issue for me as well. I have tried from thin to thick, textured, smooth, you name it. Pretty much always having the same issue, especially when I am strumming aggressively. One thing I have done that works to a degree is taking a small nail and punching some holes in the pick. It may sound a little strange but this has worked for me in the past. Hope this helps