For Christmas,I got my son(he is 7) a  Bachman Chattanooga starter set, with a E Z track system(the black one). I also bought some more track and two turnouts seperately. It´s a pretty basic layout. 4x8 ply wood with a few accesories. Since he only runs it when supervised, we have not used it that much so it seems strange to me that in such a short time it would start too run very jerky cause the tracks and wheels weren`t that dirty(I cleaned them anyway), so I´m guessing locomotive problems? Is there anything else I can do?

PD we are completely new to this hobby so all help will be appreciated.