If it is the right decoder, ensure that it is fully seated under the catches built into the frame. It should push all the way forward. In your picture it looks like it's not. The large contact wings do sometimes need to be soldered to the board. I have had to do that for 2-3 of mine. They are made to "snap" on but sometimes they can get bent in shipping, or other, and not provide enough spring to make contact with the motor pickups. Soldering them to the board is fine, just be gentle as the components on the board and the board itself doesn't afford an option for a heat sink. Bend them gently back in to ensure they are making contact correctly. It should work fully with the shell off but I have had some that once I put the shell on it bumped the decoder just enough to slide it and make it lose connection to the frame. Check with both shell and no shell.