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.

You should be able to get even a low-end derailleur to shift reliably, as long as nothing is broken. Since a front derailleur interacts with the upper run of the chain which is under load, you should shift with the minimum of force on the pedals.

I would recommend setting up the front derailleur again from scratch, since there are several adjustment points to contend with: angle, height, low limit screw, high limit screw and cable tension.

I am sure many wiser than I will chime in, and I know there are as many ways to adjust a front derailleur as there are mechanics, but here is what I (a total tyro and neophyte) do...

I completely loosen the cable, put the chain on the small front chainring and the big rear sprocket, adjust the low end stop until the shifter cage is almost but not quite touching the inside of the chain. I then adjust the cable adjuster at the shifter to mid range. Then I shift the shifter to 3 (remember the cable is still loose), and use a fourth hand tool (or small vise grip) to tension the cable and move the deraillure until the cage is centered over the large front chainring. Tighten the cable there. Use the cable adjuster at the shifter to fine tune cable tension so that the chain shifts properly between the middle and large chainring. Adjust the high stop screw to limit the derailleur travel to this adjustment. Ride the bike and fine tune with the cable adjuster as needed.

It works for me but I am a newbie so prefer advise from others.