How long have you've been offering this food?
Gliders can take a while to adjust to changes. Just keep offering it at first with the pellets and apples, then gradually reduce the amount of pellets.
If after a week or so they are still not touching it, or eating any of it, change to a different diet, again give it time for them to adjust to it.
Sometimes it takes a few tries of different things until you find what it is that they like, just because they don't touch it the first few nights doesn't mean they don't like it, it is just something different they've never had before. Also, variety is key, try to offer the fruits and veggies presented differently, cut in different shapes, put in different places (hide some, put them in treat cups), etc; sometimes that can make a huge difference.