It seems like the "b" tension screw might be out too far? It's located beside the rear dérailleur bolt and pushes on the dérailleur hanger. Essentially, in the bigger sprockets on the cassette/free-wheel the dérailleur needs to clear the sprockets with a few links to move. If you manually move the ratio to the biggest sprocket and the biggest chain-wheel then adjust the "b" tension screw in far enough (lift the dérailleur back off the hanger) so that it clears the sprocket and the chain links can turn freely. Also, is your low limit screw in too far? You should be able to manually pull tension through the cable while pedalling and move the dérailleur to the top, but not over the largest sprocket. - Cheers