will start once cut off will not start

I have a 2002 Nissan Altima 2.5 & my problem is when I start my car in the morning it will start right up with NO PROBLEMS!!! Now when I get to my destination & turn it off most of the time it won't start back up. If it won't start I have to wait about 20 mins. & it will start with no problem!!! Can anyone tell me what going on to make that happen??