It could be as simple as a flash tube, or it could be something internal.
Using gloves or a towel, I would first remove the tube on the mis-behaving light. Check for any visible fissures or cracks in the glass. If any are found, that is the likely culprit. Also, inspect it for burnt/charred metal prongs on the legs. If they are burnt, then the unit will need to be serviced, as this is a sign of arcing, which will also affect the contacts inside. Note: this is charring of the metal on the outside of the tube, not the blackened areas inside the glass. The blackened area inside the glass is normal.
If there is no charring, you can swap tubes with another light and see if the problem follows the flash unit or the flash tube. Be sure not to swap tubes with charred legs, as both a flash head and flash tube will be charred from arcing. This charring can cause arcing when swapped with a new flash or flash tube. Anything charred can "infect" anything it is swapped into.
Once flash tubes are swapped, if the problem follows the tube, the tube is bad. If it follows the head, then the flash unit will likely need to come in for repair.
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