1. remove your chain.
2. detach the cable from the pinch bolt of the rear derailleur.
3. loosen your stop for the biggest cog all the way.
4. by hand traverse the derailleur all the way through its travel and see if the
derailleur will travel all the way into the spokes unabated. If you get clean
actuation right past the biggest cog, then likely your derailleur is fine.
5. with the cable removed, align the derailleur cage cog and large cassette cog and adjust the stop screw.

6. Reattach the chain.

Before you reattach the cable, click the RH brifter through its cable travel and see if the cable pulls smoothly both out and in.

7. Reattach the cable only this time...reattach it 1/8- 1/4" tighter then the previous position shown where the cable was pinched by the pinch bolt. Disregard that you won't be able to get into your smallest cogs...not the objective. Ride the bike and see if you can hold onto the big cog with the shifter rachet in a different position while under load. If you can, then likely you have a shifter issue or you didn't have enough cable tension to begin with to hold all the way against the stop in the big cog. You may have a shifter issue anyway, but logic suggests if you hold your other cogs ok in their respective rachet positions, then a lower shifter rachet position on the big cog should work.
HTH,
George