I hate it when shop folks cannot admit they are having trouble with some adjustment and just pull the customer's chain( pun intended). Front derailleurs are just about the simplest functioning part on a bike. They are also just about the hardest part to get adjusted just right. In my shop we run into difficult Front derailleurs all the time. The problem has increased exponentially with each increase in the number of cogs in the rear. The cages narrowed as the chains did. Combined with a more dramatic cross chain problem created by more gears and sometimes it is magic that solves the problem. Or dumb luck.
I could run down some of the techniques I use to dial them in. But I admit that sometimes, what I did to fix them makes no sense. And I admit that on occaision I am unable to completely dial the rub out. But before I make a suggestion, first some questions:
~In what combo's does the chain rub? Big in the front/small in the back? Small in the Front/ Big in the back? Big in the front/Big in the back? OR Small in the front/ Small in the BacK?
~What shifters are you using? Do they have a trim feature?
~Is your Bottom Bracket the correct length for the crankset you have?
~As womble asks, is your crank or chainrings bent? That is, does the rub happen consistently or intermittingly?
Several factors come into play. Cable tension. Vertical position of the derailleur. Horizon position of the cage. Using the set up instructions will often work just fine. But often they factory speced set-up is but a starting point.