It's nearly always the hazzard switch, the reason for this is the lack of exercise the switches get, ie never get used, the beeswax that they were filled with from new goes hard, jams the switches mechanically and also stops current flow across the pin terminals inside, a easy if fiddly fix, take apart spray out clean with wd40 etc drench it well, dry out with air etc, then either put in beeswax on the switches and locking mech for the up/down action or use lots of furniture polish that contains silicone(make sure it does some don't).
tabetha