I've had my tennis tutor plus for several weeks now and have been using the heck out of it, but the past 2 days its been acting up, some balls are not being grabbed by the wheels, and all the balls line up jammed inside, sometimes 3 or 4 shoot at the same time , sometimes the balls just overflow inside and fall inside the machine. Its starting to annoy me. I use a mix of pressureless Wilson balls, and some older pressured balls. I used some light sandpaper to sand the wheels, I'm not sure if that good or not though. You guys who own ball machines have you experienced this problem and how do you solve it?