Quick course...
Exposure to UVB allows the chameleon to produce D3 which in turn allows it to use the calcium in its diet. The UVB should not pass through glass or plastic. Direct sunlight is the best form of UVB but you need to give the chameleon "shady" places so it can move out of the sunlight if it wants/needs to.
Since most insects we use as feeders have a poor ratio of calcium to phosphorous, its important to dust the insects before feeding them to the chameleon with a phosphorous-free calcium powder such as Rep-Cal.
I also dust twice a month with a vitamin powder that contains a beta carotene source of vitamin A. Beta carotene can not build up in the system like preformed vitamin A can. Excess preformed vitamin A can prevent the D3 from doing its job and lead to MBD.
If the chameleons get no direct sunlight, I also dust twice a month light with a phos.-free calcium/D3 powder. D3 can also build up in the system...so don't overdo it.
Appropriate basking temperature aids in digestion and thus in nutrient absorption.
Gutloading the insects and feeding them a nutritious diet will help too. Crickets can be fed a wide variety of greens (dandelion, kale, endive, collards, mustard greens, etc.) and veggies (carrots, sweet red pepper, squash, zucchini, sweet potato, etc.).
Calcium, D3, vitamin A and phosphorous all play parts in bone health and its important for them to be in balance. You need to look at what you supplement with, what you feed the insects and what you feed to the chamaleon in the way of those things.
Hopefully the calcium levels have been corrected since you found out that the store had given you bad information.
