I had a similar issue.
Air needs to flow from one direction to the other. Most people like for it to flow from bottom/front to upper/back. some fans are useless as far a determining which direction they blow. If it is, test your fan directions by loosly a paper towel or a klennex close to the fan. I know this sounds crazy but sometimes using your hand to feel which direction it blowing sometimes fakes you out.
Also make sure that nothing is blocking airflow out the back.
Best of luck!