I had the same problem until a few minutes ago! Do like this:You can remove the 10? screws which hold the bottom. Lift this away. In the mechanism for the arm you can easly find the lever which push the arm back to the bracket it is resting on.
Can you see the black rubber piece which is pushing the arm? This has become sticky by age and holds the arm when it returns to start position. I fixed a piece of addesive paper tape (the sort which you use when painting)on the sticky surface and the PROBLEM IS SOLVED. IT WORKS!
Best regards
Bo from Leksand, Sweden