Im sanding my car(electric hand held sander) and I have 320 grit sand paper but it works fine for about 5 minutes then it stops taking the paint off. Why is it doing this?

The paper is used up and needs to be changed out. Run you're fingers over the used piece in the sander,it will be smooth no more sand on it. Then run you're fingers over a new un used piece,it will still have sand on it. Hope it helps.