You might find that after some time your Marty's arm starts to move unexpectedly, this can be caused by spinning the arm around which then puts the arm gear in the “incorrect” position. When Marty waves the expectation is that the arm and the arm gear are in the “correct” (arms straight by the sides) position.

It's possible to spin the arm around which then causes the arm gear to be in the incorrect position, even though the arm still looks like it's correct.

Here, we can see Marty waving correctly, then after the arm is spun Marty waves badly.	To fix this issue, we have to spin the arm back to the position that the arm originally started in, for this we spin the arm until the wave is correct. A bit of trial and error may be required.
Your Marty might have alignment marks on the arm gears, if this is the case simply spin the arm until the marks align nicely.