Oh that's odd! Do you cook the zucchini and squash? Ours will pick at it cooked, but won't even touch it raw. We do feed ours avocado occasionally. It's fine, but it is high-fat so you have to feed it occasionally like with almonds. They ADORE avocado and almond, it's like treats for them. They will literally take chunks of food and throw them out of the way, scrambling to get to the avocado lol. There is a great list here:
http://www.critterlove.com/uploads/1/0/3/5/10355506/_sugar_glider_safe_fruits__veggies.pdf
It even shows the Ca:P ratios. You should keep that ratio as close to 2:1 as possible. You can do this by balancing their foods out - for example, papaya is higher, but banana is lower.
We feed yogurt when we're out of HPW, and sometimes we feed eggs. The thing with yogurt is that it's dairy and it has bacteria and such in it, plus other ingredients that are iffy (make sure you never feed them any yogurt with artificial sweeteners like aspartame). So it's okay, just alternate proteins. We give them around 1 TBSP most of the time, for 2-3 nights to use it up, then take a break from yogurt.
Our gliders favorite foods are: apple, banana, melon, cucumber, carrot, red pepper, yellow pepper, avocado, sweet potato, hard boiled eggs, plain cooked chicken, and almonds.
Also, sometimes weather, moon cycles, and stuff like that alters their appetites. Don't worry too much. But if it happens again, you should probably call your vet. Gliders can get sick which can cause appetite loss. But really, sometimes low appetite happens, just like with people.