I would start trying to "push" the gliders some.
One thing I would do is put my arm down and put the treats over my arm. forcing
them to step on my arm to get the treat. I would make the next step after that to make them climb on my arm to get to the treat. In the past once I get them on my arm I move away from cage so they are force to eat the treats on my arm as they are to far away to get back in cage.
also, picking up a glider with a one hand grab, I feel, simulates everything that is scary to them, coming over their heads from up top like a predator, squeezing them to get a hold of them. I would start with a two handed scoop. put both hands around them, then come in till they have to try to jump over your hands, and then you pick them up faster then they jump.
Alot can depend on were their biting is at. At start we have gliders that bite hard, draw blood, so we don't pick these guys up. then we have some of our gliders that may give you a small nip to me this is just a "i'm not happy with this picking up so I'm letting you know I don't like it." this usually isnt a big bite and I would just pick them up anyway if I need to. then we have some that don't bite, even if picked up and not happy about it.
If your gliders aren't biting hard, start picking or scooping them up and putting them on you. If they leave you, then wait awhile and scoop them up again.
another thing I do with scarface, she's not so happy with humans, is just put my hand near her, don't pet her, just try to have her not run away to eat her treat and me have my hand next to her. let her think my hand is just a safe thing not to be feared.