They're destined to fail unless you spend over £50. I've been through about 5 cheaper units until I bought an expensive set. Some last longer than others but the cheap/mid range one's aren't made to last.
The best of the cheaper ones (in my opinion) are linked above, if you want a good mid range set look for 'Jenning Jscale JT-2 350g x 0.01g'. You'll need calibration weights with the Jenning set.
It's probably worth just buying a new set and binning yours.