Sugar gliders nip each other, especially young ones. It is part of their behavior. I cant say what it is all about but it might be a test of being alive, looking for food, could be part of the colony dynamics such as pecking order... I have watched parents nip at baby feet and I have seen babies learn to nip from the parents.
Something that does not respond to a nip is always worth exploring further.
I know that my old momma who died a few months ago felt really bad the morning she died and all she did was sit there still, perhaps trying to stay alive or find comfort. Her offspring girl came up to her and started with a groom or a nip that went unanswered which turned into a bite and then turned into a huge gaping hole in her skin that was being ripped open by her own child. And this is the time that I found her of course and was able to remove her. She was still alive and able to attack me for messing with her, yet she let her own child eat on her skin.
Gliders almost never mean ill will, it is their nature to bite everything and see what comes of it. Now, what you do then is where the magic comes in. If you hit the animal, it will turn that nip-test into a bite next time because it will remember you attacking it. So dont do it.
Remember that it is simply acting on instinct, not anger. There are various methods for responding, collecting, restraining or conditioning the animal and it does not involve roughness or anger on your part.
A nip can hurt but it doesnt break the skin. A second nip often will because they are continuing their exploration of the "thing" and advancing their response.
When a sugar glider receives a nip, they usually squeak, push back, nip back, fight or run away. This ends the nip/biting. This tells the nipper that what they were nipping at is alive and/or perhaps not going to fight back for dominance.
When you receive a nip, you can
mimic a glider's behavior. Bump the nose, squeak or go "TSSSST", withdraw. Keep an active eye when working with a nipper. They will always come in for a nip so you will always preempt that with a nose bump or a sound. Nip them first instead. Work to establish dominance in a way they understand and after some time they will begin to remember the pattern. Present your tougher knuckles or wrist instead of your finger tips. Tease a nip/bite then respond in the sugar glider way.
My conditioned way of presenting myself to all of my gliders is by an immediate and light nose bump. This is how I say hello and they are used to it. This gives them a conditioned greeting, a second to process that it is me and safe and then I can pet them or grab at them. They never bite from nose bump hellos, even the few biters that I have. The ones that I can love on usually get a nose bump by my actual nose when I say hello. I will stick my head in the cage and bump noses just because I am stupid that way and have to get my cuddly-wuddly on. It's amazing how much wetness is on a glider nose.
I have worked with biters in the past that have gotten 'better' under various conditioning techniques. I have literally seen them come right close in for a bite with open mouth but then stop after either realizing it was me and/or also recalling the conditioning. That is a sweet moment. Reward it. Gliders respond to treating.
I have a blind boy who has never seen and he always bites at me when he smells me. I have recently gotten him to improve by feeding mealworms by hand. He has to smell me first to get a worm. Now when he smells me he thinks he is getting a treat and he looks for that over biting me. He gets a nose bump greeting, and then he will crawl up my hand looking for a bowl of mealies instead of biting at me. I have replaced his fixated fear of me with a positive memory of treats. It isnt the end result I want but it is an improvement that is worth reinforcing.
So you can succeed or improve here but you need to think about the
right way to do it and never lose it because the sugar glider will remember when you are mean to it and that ruins everything.
P.E.T.E.R: Patience. Effort. Training. Enforce. Repeat.