I have seen this symptom many times but it is not the neon bulb that is failing but the main capacitor that powers the flash. Neon bulbs have a long useful life, far longer than that of capacitors. When the neon bulb no longer lights or is slow to light it indicates that the capaacitor is no longer capable of holding a full charge. The unit may still flash but not at maximum output. The condition will continue to get worse until the unit no longer will flash.