It's most likely the actuator, I have replaced 2 so far and am not happy as they cost $200-$300
because it needs a 1600 vee dub engine in it to make it go faster to have lawn mower races
your electric motor(I assume) probably wont have enough torque, That is on a Riding type mower, not a push mower, it might work on a push mower as long as its not self prepelled, because your motor probably still wont have enough torque!
replace it
bad bearing
starving for fuel
get a new one
release the belt tensioner bar on the right side of the deck. you can do this by hand, it is the rotating bar that is held back by a catch on the deck. at the front of the deck where it is attached, rotate that bar downward to release it. at the rear attachment points on each side, you will just pull out on the round metal peices that stick out of the side of the frame. if they wont pull, you may need to lift up on the deck lightly to lessen the tension. pull the belt off that connects the deck to the mower and slide the deck out sideways. once you do it one time you will be able to do it in five minutes
dirty carb bowl or water in the carb
Cos you're a douche
it is just not working. OK
Try pulling out the spark plug and clearing the cylinder
Take it to a shop