After not riding much since I was a kid, I decided to try and get back on my decade old mountain bike a while ago. I had it serviced (twice) at the shop at which it was bought, but problems with chaining shifting still remain. The chain never completes a shift between any of the 3 chainrings, and occasionally falls on either side. The shop I took it to didn't really have much to say about this, only sheepishly that "you expect this sort of thing", but wouldn't elaborate.

My first thoughts to this sheepishness were: are the groupset components crap (Shimano Alivio front dérailleur), or am I putting too much stress on them. I weigh 230lb/100kg and the shifts are somewhat better if I barely put any force on the pedals.

So my question is basically; should I blame the components, blame the shop, or accept that I personally can't expect decent performance from a non-heavy duty machine.