My 2007 chevy cobalt ss is having trouble starting?

For about a month now my car has been having trouble starting. it will roll over about 3-6 times and then finally start. then once ive started it and turn it off it will start right away. So basically once it's been sitting for a couple of hours it has trouble starting. I just replaced the battery less than a year ago and we went to checker and they said my battery and alternator was 100%.