P180 mini 120mm fan problem

Hi!
On my p180 mini the rear 120mm fan does not spin up when the speed control is set to "low". If I change to "medium" or flick the fan with my finger the fan will spin up and then work fine.. What is the problem and how should I fix it so the 120mm fan starts up at "low" speed when the system is powered on?
I would like to avoid running the fan at "medium" or "high" because of the increased noise.. Also I can't manually keep switching back and forth the fan speed as the chassi is mounted into a kind of a computer desk and it's not possible to reach the speed controller/switch without a lot of hassle.

Otherwise the chassi is great

Thanks!