There is something you can try... And I repeat try. There is no guarantee this will work for you, and I don't recommend it (but it does work if done right).
Take the motors out and make a test loom.
Flick switch, Wire, Connectors.
Then run those motors in a glass of deionised water till the water goes black, Dump the water and do it all over again till the water starts to come out reasonably clear.
Re-lube the bearings top and bottom, Put on some new brushes if needed.
This will clear the crud out of them, All the carbon and brush deposits that get into the bearings, magnets and caps.
Afterwards, you'll either have dead motors, or ones that run like new.
Just be sure to allow the motors to fully dry before you install them back in the guns, and lube again before they go back in.
Most of the supermarkets sell the water you need for about 60p a litre.
https://groceries.asda.com/product/car-maintenance/carplan-deionised-water/479358