There's more to truing tires than just the tires. The first thing to check is if the axle is straight, and if the wheels are on the axle straight. If either are too far out of whack then the car will hop especially if you're taken the traction magnets out. One should also make sure that there is not too much play in the bushings - either the axle being loose in the bushings or the bushings being loose in the chassis. Next, check that there is no molding flash either on the plastic wheel or inside the tire. As SCRJohn and guinner16 point out these issues can prevent the tire from being properly seated on the wheel.
When I started in the hobby I used a drywall spatula with two grits of sandpaper glued on each side. I'd face the car going backwards on my power base and gun it. Not sure if you can do that with digital.
Once I had been in the hobby for a few years I decided that I needed something better for tire truing. A few guys in my club have a Hudy which works great for setscrew wheels. Since I have a lot of cars in my collection and like to keep the stock rear axle assembly whenever possible I bought an RSMII tire trues from Germany.
There is a similar product called 'Tyre True'. This type of machine works best with aftermarket urethane and most stock rubber tires.
This is just an overview but it will give you an idea of what's involved to set up a car with plastic wheels.