Need help removing tiles

After 2 years I've finally begun to tackle the loose tiles in my shower. After taking down a considerable number of tiles I discovered the wall behind is mortar rather than green board or cement backer board. Also the wall is in very good shape so I am attempting to replace the tiles that were loose. My problem is some tiles did not come off the wall completely. I've tried to use a Dremel tool to saw the grout and remove the tiles but this is not working. Can anyone give me advice for removing these tiles without destroying the mortar it's stuck to?