You may find this web page helpful
http://www.suzsugargliders.com/educationalarticles.htm
More joeys are rejected at a younger age than 4 weeks OOP. If the joey has not been injured, it may be that mom is just not producing enough milk.
What diet do you have your gliders on? You may need to make some changes in what you are feeding so that Mom can produce enough milk.
In the meantime - read the information on the link above on rejected joeys to learn what and how to supplement the joey some at night.
The joey is probably old enough to drink some formula from a small container - many people use the cap from a milk container or similar size 'dish' for joeys. By 6 weeks OOP most joeys are following parents to the food dish and beginning to eat the staple and some of the fruits and vegetables you offer the parents.
Another possibility is that the joey has health issues and the parents know what we cannot see. A vet check for both mom and the joey would be a good idea.