The perennial question. The answer is practice and trust in the diaper. If you can't pee lying down without forcing it, pee sitting down, or lift yourself onto your knees and pee in that position. Try to relax and pee lying down each time first and then switch if it doesn't work. The other thing is you need to get used to peeing in various lying down positions where you know the diaper won't leak. It makes you mentally at ease. Like, for me I can only pee lying down when I'm pointing down because I leaked out the top or sides in other positions and my body won't let me go because of that. Then just keep at it. It took me several months before I could go lying on my stomach.