The capacitor used as a V dropper is a metallized film and should be X2 rated for use across the AC line. But almost all of the small power supplies I have looked at use regular metal film capacitors not rated for use on the AC line. This is not much of a problem where I live in the US because our line voltage is 120VAC. In other countries where the line voltage is 220 to 240VAC the demand put on the voltage dropping capacitor is much higher.
As the line voltage spikes hit it, the metallization gets ablated and the capacitance drops little by little until it’s too low to function. Measuring it will show the problem. A new one should fix it.
However the problem is it’s getting these line voltage spikes, which should be eliminated by a capacitor across the AC line. But then that may suffer the same problem if the spikes are nasty enough. Apparenly if that’s the case, then the incoming AC line needs some inductors in series. All these things should be in decent quality equipment – they’re usually in switch mode power supplies – but that’s just the way the cheap equipment makers try to get away with making it cheap – they put jumper wires across where the inductor(s) is/are supposed to be.
