Zapper fail / wear & tear?

For the first time in 7 or so years my (Clark) zapper is not 100% effective in curing the cold or flu I seem to have. Usually I zap and the sickness is gone soon after. This time although Im certainly not as sick as I would be if I wasnt zapping, its been 3 days and my throat is still sore and Im coughing a little with slight fever & just not feeling as well as I should be!
Can zappers wear out over time or am I just unlucky this time? I had another Zapper of the same model which broke, but it was an obvious break where the lead to the battery broke loose.
If its time to buy a new one, what is the best type for treating colds and flu? I had a Terminator for a while but didnt like it as much. Im wondering what experiencies people have had with the Beck for colds & flu?
Anyone? :-)