All kinds of things can cause a stuck fermentation.  Pitching more yeast
is certainly a good first thing to try.   If the issue is due to the
yeast prematurely flocculating, this is a solution (and remember it will
never hurt to overpitch either).

If the problem is caused by a lack of nutrients (and in commercial 
operations, zinc deficiency is a common one), adding some commercial
"yeast nutrient" mineral mix is also worth a try and will do no harm.
A homebrewing store should have some.

In commercial facilities it is common to take a couple dozen gallons
of wort that _is_ properly fermenting and dump it into the stuck batch.
You're adding both yeast AND some amount of nutrients when you do this,
but it's not always that effective.
--scott
-- 
"C'est un Nagra.  C'est suisse, et tres, tres precis."