Originally Posted by FBinNY

If it always used to work, but now doesn't, that's different than if you installed a new cassette.

If it's a new cassette, probably, as AL1943 suggests, you have the wrong lockring.

But if it's a new problem on an old setup, odds are that the outer limit screw is too far in, as Stan suggests.

But don't just back ff the limit and forget it. Limit screws don't magically tighten themselves. If the outer limit is now too far in, it's an indicator that the bike was dropped and the hanger got bent in a bit. That's important because it means the inner limit is now too far in by the same amount putting you at risk of over-shifting the chain or RD into the spokes.

Back off the outer limit a bit and see if that solves the problem. Then check the inner limit by shifting to low gear using direct pressure on the lower body of the RD. You want the limit adjusted so it just barely can complete the shift. Now shift back and forth to low from 2nd to low and back off the limit just enough to allow a reliable, crisp shift and no more.

BTW- at some point you'll want to have the hanger checked, but it's probably close enough that you can put that off until the next tune-up.