You can train them out of it, but you have to be really vigilant and consistent (and they may relapse). It may have to do with how they were raised, if their parents went in the pouch, they think it is normal and it is just a behavior that you have to teach them they don't want to have. If you have another glider that dislikes a dirty pouch, that helps too (although it can backfire and they can both start going in their pouch
).
I have a boy that I got as a joey and he
always wet his pouch, but his mate is crazy clean and HATES it when he goes in the pouch, I've heard her crab at him several times over it and have seen her smack and bite him a couple of times right after he did it too, so I decided to help her out.
Have several clean pouches ready, you may be changing them frequently for a while. Get them up a couple of times a day and stimulate them to go potty away from their pouch. (To stimulate them, get a moist q-tip and very gently blot or rub the cloaca until they go potty...do this
every single time you get them up, for play time, during the day to bond,
every time. They will get used to it and start going as soon as they get up, but if they don't, stimulate them to.)
Check their pouch several times a day. If it is soiled, change it immediately, every time. If they get used to sleeping in a clean pouch, they won't want to mess it up. It will take some time, but they will get the hint. None of my 6 soil their pouches. Every once in a while, something happens and one of them starts (it's almost always my boy that started out that way), so we go back to square one. But I check pouches every day anyway, so it never lasts long.
Several of my gliders will even chitter at me and pull at the zipper to let me know they have to go potty if they are in the bonding pouch, so I let them out and they will go in my hand on a kleenex then run back to the pouch. (They hate a dirty pouch THAT much).
They earlier you start, the better they are about it, but even older gliders can learn to love a clean pouch with time and patience.