Tires going bad.
I race on and off - about once per month in the winter and not at all in the summer. It is a good idea to take the tires off your car and store them in zip locks and in a dark place. I have left tires on the car over a summer and no matter how much you true them, they are done. You may get some practice out of them, but you'll need to soak them down good the night before you intend to use them - just let them air dry.
Over a month, you won't see much of a loss in traction, but rubber or foam will definately lose stickiness in the air and light. I run my old crap in practice and club racing. When I go to a bigger race like the Carpet Racing League or a Regionals, I'll buy a new set or two.