I have the same problem with most brands, mediums and large.. I have a bit of a belly which doesn’t help. A ProtecBrief helps a lot, but they’re pricey and kind of warm, so I’m interested to know what others have found too.
If it’s a 4 tape plastic backed diaper. Try tapping it at different angles. Each diaper is going to fit a little different depending on the cut and size. I find myself in between a medium and a large if the diaper is made outside the US. The taping is super important in positioning and how tight you tape it on. The angle of the tape has a lot to do with it as well. The bottom tapes for work at a slight angle up. Top tapes are a slight angle down. Don’t tape the top ones too tight. This makes them want to slide down below the belly fold if you have one. It takes a little bit of patience to get it right. You may try a diaper that has a landing zone. MegaMax from North Shore works great for figuring this figment out. They will send you samples if you live in the US.