Firstly, don't allow yourself to get frustrated (easier said than done). Gliders are exotics...just a step away from the wild and their behaviors must be understood in order to make it through the bonding process. They also will feel your frustration, and any possible trepidation that you have in knowing a bite is likely coming.
Your friends have most likely gotten joeys that had been well handled by the breeders. Regardless, it is often (but certainly not a hard and fast rule) seen that it can be easier and take less time to bond with the joeys.
What sort of out of cage area do you have set up for them? Sometimes the size of the area being larger may contribute to them delaying checking you out and getting brave enough to climb on you, etc.
The length of time is also a factor. How long have you had them? Very new gliders are going to take awhile to get used to their new surroundings as well as learning that you can be trusted. That being said, each glider has their own personality and some bond faster and easier than others. Each also bonds to different degrees. Of my two sisters, they are both very well bonded with me and we have firm trust in one another...but there is a different level of how much one prefers to 'be' with me over the other when they are out of the cage.
My newest addition is behaving like a bratty toddler. He prefers to bite a finger that is not holding the treat...as if he's saying, "How dare you touch my nommie, but since it is here I'll go ahead and eat it."
They are acting out of fear and testing their boundaries by nipping at you to see if they can keep you at what they currently feel is a safe distance by this behavior. By keeping consistent, not getting frustrated and not allowing them to succeed in you pulling away or giving up...the behavior in most cases clear up over time. The tried and true method of mimicking their own "Psst" sound when they bite is one that can always be used to speak to them in their own language that the biting is not acceptable.
Keep the faith!