Hi Healther,
It could be something like nighttime seizures which is one of the top 3 common reasons for tongue biting in your sleep, these seizures are usually a tightening of muscles accompanied by violent twitching but involuntarily biting of the tongue during sleep is definitely suggestive of seizures.
Carbamazepine is usually effective in the treatment of nocturnal motor/dystonic frontal lobe seizures, so if the Carbamazepine is no longer working for you i would strongly suggest you contact your treating physician to discuss your potential solutions.
Hope that helps....JJ