I have never had a toothed belt fail on a 24/25, 99% of time for me it is head motor or yoke cable
For the brush to stop when it hits the floor is unlikely to be the PCB although this doesn't rule it out! It could be a number of things.
*Internal cables - are prone to snap and break, will cause the brush to stop in certain positions, for example it may work in mid air but on the floor the cables are bent in a different position so not making a connection and not working.
*Worn motor - Worn motor or otherwise end of life could struggle to run and may well spin brush in air but once resistance is present (such as brushroll on carpet) it no longer can handle and shuts off
*Worn motor spindle - Worn motor spidle could mean that althogh the motor is good and running, once the brushroll has resistance (like touching the carpet) it could start to slip.
*Loose belt - A loose belt would be the same as a worn motor spindle - works in mid air, but once it has some resistance, will start to slip
*PCB- Unlikely but could be a component failing that causes the motor to not run as effieciently as it should, unlikely but possible.