You shouldn't need to adjust the high/low stop screws once a derailleur's correctly fitted.

What you almost definitely need to do is tighten the cable - they stretch over time. The easy way to do this is at the barrel adjuster on the downtube, or you could just open the clamp and pull more cable through.

If you've had those cables on for a long time, it's probably a good time to check them for frays.