Well, here is my opionon when it comes to peeing in diapers this is coming from a male, at first it takes awhile to be able to pee in a diaper, after a month or so, you learn that its okay, and your body doesn't prevent you from peeing while in a diaper, the problem with peeing while laying down is this, by any chance is your subconsious thinking that it is wrong, or its someting new so it thinks that its not supposed to do that? so it prevents you from peeing? is your diaper too tight sometimes when its too tight, it blocks your pee from coming out? (this is from personal expirence)
Tell yourself that its okay to pee while laying down, and just keep trying, eventully you'll learn that its okay, and your body will be like yep, he is in a diaper, its okay to pee, you could be subconsiously thinking that, its gonna leak, or something bad will happen, simply because you aren't used to peeing the diaper like that.
Simmlar thing happens to when when I want to pee my pants(don't ask me why, its when i regress) I cannot do it, I Just can't without telling myself its okay, or trying to convince myself I'm already wearing a diaper, it could just be your brain has some self doubt? and even then, sometimes I only empty half my bladder, or none at all.
Hope this somewhat helps.