Maybe try hitting the stomach with the edge of your foot or something? From what I can tell, DMs can occur from pressure but a lot of the time it's just the angle that the joint is hit at: if your foot hits so that it wedges between two joints then it will split them apart rather than if a flat side of your body hits the center of a joint. (obviously this is pressure too but it's caused by the angle of the hit and not exactly the force of the hit itself)
Of course I don't want to sound pretentious, I feel like you would already know this...
Anyhow Unless if by stomach DM you mean that you are trying to split the Uke with YOUR stomach and not that you're just trying to split the Uke's stomach, I would say try kicking him with the point of your foot, or maybe your knee, or some other pointy surface other than your hip.
Just trying to be helpful. ._. Trying at least.