I am new to guitar tricks as well, this is actually my first month being subscribed, i have had the same issue with my pick slipping out of my finger when strumming, something i found that has helped me a lot was i switched to a lighter pick that was a bit more on the flimsy side to where the pick will give a little when strumming, it has helped a lot, and i added a lot of practice time just trying to get my strumming technique down, just thought i'd add what has helped me with the same issue.