All cars spur gears moving in and out Even traxxas spur gears move in and out alittle bit. You need to mesh the gears by the paper methode. Especially if this is your first car. other wise you'll go crazy thingking you have it right. After you've done it a billion times like I have you'll get the feel for it. Trust me if your not doing it y the paper methode then you more likely than not had the mesh too loose or too tight. I would gather to think too loose and you stripped the tips of the teeth off.

To do the paper methode you stick a piece of paper between the spur and clutchbell then work them back and for pushing the motor in so that the paper meshes nicely between the spur and clutchbell gear with out ripping the paper. then while gently pressing the engine and clutch bell against the spur (gears evenly meshed, not crooked) tightend down the engine screws on the bottom. Turn the spur gear to get the paper out and you should have a perfect mesh. I always forget to save a picture, but if someone has one that would be great if they could post a pic.