Just unscrew the tension screw on the bobbin case....be careful not to drop that tiny screw! Then lift off the tension spring. You might find lint/thread under there. If you do, that was your problem. After I take the spring off, I usually soak it (& the spring) in alcohol & clean with a q-tip. Make sure it's good & dry before reassembling.
By the way, don't put any oil between your tension disks....I think you said you used SMO to clean the tension. It can leave a gummy film that will just get stickier over time. Again use alcohol.