How to avoid discomfort with extended use and when to size up?

So I got a basic set of plugs small to slightly larger than normal large. I've been able to get the 2 largest in with very little discomfort but after laying around with it in 1 it starts to feel like it's slipping out and 2 it becomes uncomfortable not painful just kinda annoying.

Any advice?

Also I love y'all for recommending the lube injector it has made play time a lot easier and less messy.