The old Mosquito Moulds ones used to have a bad reputation as they have 8 very small bearings in them, which have to be compressed when you put the grenade back together. One of mine stopped working so I stripped it and worked out how to reassemble them. Once you know how it is fairly straight forward (a lot easier then tackling a gearbox!) - not sure if the ones you have are made the same way, but the MM ones have an O ring inside the mechanism which was the one that always went (or got nipped when resetting).