Sometimes it is just a matter of getting the setting on the "b" screw correct. I didn't like the way the SRAM X7 RD performed on my recumbent trike with an 11-36 cassette so I switched to Shimano. I used the guidance offered by the Sheldon Brown website to buy the right Shimano derailleur. They warned it would not work right with the b screw sent by Shimano but switching to a longer b screw would allow the RD to clear the 36. They were right. It's been in use now for 6+ years and shifts perfectly. Before you take it back, try adjusting the b screw.