There are about three primary causes of seizures in bearded dragons, calcium deficiency (or lack of vitamine D3 either due to improper lighting or insufficient supplimentation or a combination of both) either from not dusting crickets or offering greens that lack calcium or are very low in calcium, or greens with high levels of calcium binding agents, such as spinach.
Lack or not enough of other nutrients, besides calcium can also cause seizures.
Too much heat in the cage can cause seizures, especially if there is no where for the dragon to go to escape the heat. (ie basking spot way to high, above 125F with no cooler area to go, no cool end of the tank at all and ambient temps are way over 100F)
Toxins also can cause seizures. Toxins could be in the food being offered, (ie wild caught insects who have been in contact with insecticides,chemical fertilizers etc), fumes from toxic substances in or around the cage or consumption of poisonous plants or insects.
Neurological disorders or genetic defects are also known to cause seizures.
Pretty much the only thing you can do yourself, is make sure the cage setup is perfect for bearded dragons, make sure no toxic substances are in or near the cage (paint fumes, poisons used to kill household insects, etc) and you are using only captive bred insects and wash all greens well before feeding to your dragon or crickets. Check to make sure your basking, ambient and cool end temps are within acceptable ranges with a digital thernometer or temp gun.
To truly diagnose what is causing the seizures, a vet inspection will be required. There is only so much guess work we can do with a description of the cage setup and feeding regime on what could be causing the seisures, as so many different things could be the cause. As you indicated your dragon is growing normally, it is not likely to be a fault in diet or cage setup, but it can easily be any of the other reasons and these can only be determiend by a qualified reptile vet.
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PHLdyPayne