Limit screws will only appear to have an impact if you're at the limit of the derailleur's movement (either full tension or no tension). If you adjust them when the derailleur only has partial cable tension they'll appear to do nothing (though you will still be changing the limits, but likely making the problem worse!!!).
I'd suggest following the instructions on the Park Tool website (clickety click
). Start from scratch... release the cable, set the limits, then set the cable tension.
If you're still having problems with it, take it to a bike shop. If it is a faulty derailleur, it's a hard thing to diagnose over the internet, but easy enough in person.