cause it happened to me once. driving an inductive buzzer load.
i changed 3 relays, no luck.
put capacitor as well as diode across relay, no luck.
isolated power supply, still no luck.
isolated with optocoupler, still no luck.
what i noticed with one of my relay(which has a transparent casing), there is a spark whenever relay activates, i figure out that inrush current is making my relay stop and pic to reset. i put a series NTC thermistor, since initially it exhibits high resistance which is ideal to stop inrush current, in series with my inductive load, and all problems gone, circuit worked flawlessly now.