I have an issue with this also with picks sliding out of my fingers. That's why I designed my hybrid thumbpick / flatpick thing. I'm not going to do a sales pitch but if the OP wants more information just shoot me an email or click the link in my signature.
One thing I have found that really helps me with a standard flatpick are the sticky dots from Guitar Moose. Stick them on a pick and it won't slide around in your fingers. Also if your pick wears out, you can peel it off and stick it to another pick and keep going. I buy these things in bulk and probably have about 30 unopened packs in my shop. They're that good. Here's the link.
https://guitarmoose.com/shop?olsPage...kydots-10-pack