Make sure that they are getting the right amount of power because some cases have little resistance switches that control the power to the fan (i.e. with the NZXT Phantom when you switch between the three fan 'speed' settings the lights on the LED fans will dim accordingly as a result of less and less power being delivered to the fans) so don't blame the fan performance straight away; unless of course the fan is rated as such by multiple people.
-SP