I had the same prob on trhe passenger door.
The actuator motor is encased in 2 half shell plastic casing. The halves seperate on the join and open slightly causing the 'geared flat slide' that the little motor en gauges to slip off it's track thus you hear the motor running but the door doesn't open.
I tried repairing it by drilling holes and fitting screws to hold it tighly together but it lasted a few days only.
I ended up getting one on ebay.
If it is your problem then
There is a seller called 'Bundene ?or Similar have a look at his ads, and PM for aprice if he doesn't list one. He is very reasonably priced.