Lithium Stopped Working

During a particularly bad cluster cyclea couple of years ago  I was prescribed lithium. I initially had little faith in this as I had read many sufferers experienced no benefit. However, for me it was a miracle worker quickly eliminating all day time attacks and leaving only a couple of night attacks. This all went well for almost 2 years but then the lithium stopped working. Has anyone else experienced this and if so if you have a break from lithium will it work later if reintroduced? I am particularly grateful to the advice given by Mike some time ago to take a sumatriptan tablet before bed as this mostly lets me have a restful and pain free night. I know we are all different and what works for one doesn't necessarily work for another but sharing experiences and trying different things can be very beneficial. The best support is knowing you are not alone. I would be interested to hear if anyone has had lithium stop working but being effective again after a break (how long).