Yes—this is a real, if somewhat uncommon, maintenance issue related to the electrolytic capacitor used in many flashes. When left unused for long periods, the capacitor’s dielectric layer can degrade. Mild cases may show up as slower recycle times or sluggish charging at first, and repeated charging/firing can sometimes restore normal behavior. In more severe cases, the dielectric can break down enough that the capacitor shorts, which can lead to failure.

That’s why some manufacturers recommend “re-forming” a rarely used flash by switching it on for around 10 minutes every few months, or at least every 6 months. One answer specifically notes leaving it on without firing.

In practice, many people never see a problem even after prolonged storage, but the recommendation is reasonable preventive care—especially for older or rarely used units.