Using one particular wormer exclusivley can over a period of time lead to wormer resistance.
In every population of any creature there is likely to be a small percentage that will have a natural resistance. By killing off all the none resistant ones you are left with the remaining resistant worms. Then these breed without any competition and over a period of time you are left with a complete population of resisitant ones.
This is why in horses at any rate, its advisable to vary the type of wormer used. A worm is a worm, is a worm, so surely this advice holds good with chickens as well.