Starting it up

So lately if I am at work and I drive home which is about 25 mins and park the car and get out and get the mail, the car has trouble starting. It wants to turn over and generally does, except today it would not turn over at all. After waiting about 30 minutes it was fine and there were no issues. It seems if I drive for more than 20 mins and then stop the car and then try to start it, there are starting issues, but if I drive down the street and run into the store and get back in, there is no problem with it starting. Any ideas?