I agree a vet check to find out if there are mouth problems preventing them from eating well or parasite issues preventing them from absorbing nutrients would be a good place to start.
If no physical health issues are found, then some changes in diet - or how you offer the diet may be in order.
Picky eaters on HPW has always been a topic of discussion. Peggy even has a full page on the CritterLove web site on dealing with picky eaters.
My gliders would not eat BML - I feel it is because of the large amount of gritty calcium supplement it contains - which is probably a bit like eating food laced with SAND.
Some gliders like their fruits and vegetables coated with the staple. Others prefer the staple served separately. If you are using one presentation - switch and try the other to see if it makes a difference.
If you are feeding one of the combinations of many fruits and vegetables each day - try offering only one or two fruits and one or two vegetables each night. I find using a mixture allows my gliders to always eat their favorite first and NEVER eat some items. When I offer only one fruit and one vegetable the will eat what is available on the plate if they are hungry.
If you have not already tried my GliderKids Feeding Plan - that might be another Diet to try. I have several recipes in response to OWNER's preferences - some want to feed less honey, some do not want to offer chicken, others do not want to use eggs etc. My recipes contain less water so the staple is a little more concentrated and allows a smaller serving only 2 teaspoons per glider to provide the nutrients your gliders need daily. It is a little higher in protein than the HPW diets which may help your gliders even if they do not eat the full 2 teaspoon serving at first.
Here are printable instructions for the GliderKids Feeding Plan:
http://www.gliderkids-diet.com/GKSFeedingPlan2013.pdf
By only offering 2 teaspoons of the staple, the gliders very small stomachs have a bit more room for the fruits and vegetables you offer with it. Most people have told me they no longer have picky gliders on the GliderKids feeding plan.
I also find that my gliders do better when offered one fruit and one vegetable each day - but a different combo each night so they get 7 different fruits and 7 different vegetables each week.