I would like to add my answer as I don’t see it yet.[br]
If you follow the link to the disassembly instructions or if you have taken the mouse apart already, you will see that there is a metal tab that is near each of the black magnets on the rollers . This is there to measure the resistance difference of the magnets so it knows that the roller is moving.[br]
In my case, the tab was physically touching the ‘down’ roller. Well, if there is no gap, then the mouse cannot tell if that roller is moving.
TLDR: make sure the metal tabs next to the black wheels on the roller bars are not touching the black wheels. If they are, simply bend them back slightly.