Well first off if i understood correctly it sounded like you said the truck doesn't roll as easily as it used to. That sounds like something could be binding but there isn't much i can tell you about that other than you have to start looking at everything and see what is binding.

You have to start looking at the simple things first and go from there. First make sure the battery is charged in both the transmitter and the car. Then try resetting the esc. If you can't figure out how to reset it call the manufacturer up to find out how to do so. You won't find instructions on how to do that in the instruction manual that comes with the truck because the esc is a seperate part that doesn't come with the kit. You need to either find somebody who owns one to help you out or like i said contact the esc manufacturer.
If resetting the esc doesn't help, then you have to go from there. I'd take the motor out and keep it hooked up to the esc to see if the motor turns when i give it throttle, then see if the gears, etc are binding.
BUT if you are not mechanically inclined at all and if this seems to daunting for you then all i can suggest is bringing it to your local hobby shop. Personally pulling stuff apart on the r/c is half the fun but if you aren't into it thats up to you.
I don't know how you adjust one of those speed controls so i can't help you out there.

Mike