breaking cam belts in td200's

my td200 periodically pulls teeth off the cam belt i.e. every six months or ~5000 miles. on stripping the engine we cannot find any faults we gave replaced the injector pump, and have come to the conclusion that it is something to do with the cam shaft. the only fault we can find on stripping was a couple of tappet sliders inserted the wrong way round. these look symmetrical is it important that they go in the right way round?
is there anyone out there with any advice/ideas on what might be wrong with the engine?
thanks

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thanks mate

Loss of tension in the belt can lead to stripped teeth, badly set valve clearances can lead to issues in some circumstances, is there any indication of a foreign body getting into the timing chest?