Tongue shooting problems are usually from MBD or dehydration as Brad said....but they can also be caused by an injury or an infection.
For those of you who are having that problem with their chameleon, once you have ruled out infection or injury, I would recommend that you review your husbandry.
Is the basking area within the appropriate temperature range? Digestion and thus absorption of nutrients hinges on the chameleon being warm enough.
Does your chameleon have a full spectrum?UVB light or direct sunlight (the light from either should not pass through glass or plastic)?
Do you gutload your insects with a suitable diet?
Do you dust your insects most of the time with a phosphorous-free calcium powder before you feed them to your lizard? Insects have a poor calcium/phosphorous ratio and it needs to be corrected.
Do you dust with a vitamin/mineral powder a couple of times a month lightly? (I always use one that has its vitamin A from a beta carotene source because it can't be overdosed like preformed vitamin A can.)
If your chameleon gets only artificial light, do you dust the insects with a D3/calcium powder lightly a couple of times a month? Caution is needed....vitamin D3 from supplements can build up in the system. Vitamin D3 produced from exposure to UVB lights or sunlight can't because the chameleon's system doesn't allow it to happen.
Is your chameleon well-hydrated?
MBD can not only cause the hyoid bone (the bone that the tongue sits on) to be weak, it can cause the muscles to be weak too. Calcium. phosphorous, vitamin D3 and vitamin A all play a part in bone health. If your chameleon has MBV then its important to bring back the balance....and then with the proper husbandry it should stay.