Regardless of why your chameleon is falling, you need to correct the supplements. The insects should be dusted (not heavily) with a phos.-free calcium powder before being fed to your chameleon at most feedings. This is to make up for the usually poor ratio of calcium to phosphorus found in the insects.
Its recommended to dust twice a month (lightly) with a phos.-free calcium/D3 powder. This ensures that the chameleon gets some D3 without overdoing it and leaves it to produce the rest from its exposure to UVB.
D3 from supplements can build up in the system and cause problems so you don't want to overdo it. D3 produced from exposure to the UVB should not build up as long as the chameleon can move away from it.
BTW...what UVB light do you use and is it a compact, spiral, long linear tube light or what?? Does it pass through glass or plastic?
It is also recommended to dust twice a month with a vitamin powder containing a beta carotene (prOformed) source of vitamin A. If the chameleon needs it you can dust once in a while with a vitamin powder that has a prEformed source of vitamin A...but be aware that excess prEformed vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD.
Gutloading/feeding your insects is important and its important to provide appropriate temperatures to aid in digestion.
Calcium, phos., D3 and vitamin A are all important nutrients in bone health and they need to be in balance. You need to look at what you feed the insect, the chameleon and what you supplement with to balance them.
As for the falling, there could be a number of reasons for it. If it continues, I would recommend that you take it to a vet.