This could be a result of a a few different problems: (1) Make sure that you're driving the truck within a reasonable range. If you're too far away from the truck, it won't respond as well or at all. (2) Check to see if the battery in the controller and the battery in the truck are charged. If they are low and/or dead, the truck won't go full speed. If the battery won't hold a charge, consider replacing it. Check out the guide here. (3) Is the antenna on the truck damaged at all? If it is, check out the guide for replacing it here. If none of the above works, try using a different controller on the truck. If it problems disappear, then it was a problem with the controller. If not, then the problem is narrowed down to the truck. Hope this helps!
Make sure that all the wheels can are spinning when you accelerate; there might be dirt or something else keeping the tires from spinning correctly (since it's only turning right it's safe to assume that it's one of the right tires). If the tires are fine, then something's faulty with either the controller or the motor. Try using a different controller for your truck and visa-versa to figure out which one it is, and replace the faulty part.