I recently had similar problems with an SD257 and it became obvious that the motor was struggling to turn the blade and not kneading the dough properly. This was due to a faulty motor run capacitor. It cost less than £5 to fix.
If you are up to the task, and are OK with soldering, remove the hinge screw and take off the lid to make things easier.
Remove the 5 screws around the top of the baking tray and wriggle things around until you can remove the control panel section. Pull out the connections from the main pcb and set the panel aside, then remove the bezel.
Squeeze and remove the clamp around the mains cable so that it can be fed into the body freely.
Undo 4 screws holding the feet and remove the entire works, feeding the mains cable through as you do so.
Remove 1 screw and the plastic pcb cover. You will see a 450v 3uF capacitor on the board. If you can find a replacement with the right dimensions, just change it, but I found it was easier and cheaper to get a stud mounting motor run capacitor with flying leads. Drill a hole in the lower flange (clear of the motor!) to mount it and, after removing the old capacitor, run the leads up to the vacant holes in the pcb. Tie them in with the other wires and it all looks very neat.
Put it all back together again.