I would suggest checking three things. First, the electrical connections at the fan as well as at the fuse/circuit breaker panel for the 12-volt accessories. Second, remove the interior screen and carfully clean each of the blades (for safety be sure that power is turned off) - - then use a can of "canned air" (sold for cleaning computers) to blow away any dirt around the shaft and housing. Third, apply no more than 3 drops of a light machine oil to the shaft. When one of the fans slowed a bit in my Overlander the three steps above restored it to perfect operation.
The warranty on the moving components of the Fantastic Vent is two years. The acrylic dome carries a lifetime warranty as per the following information from the manufacturer's site:
Fan-Tastic Vent provides a two-year limited warranty (lifetime warranty on exterior dome) against defects in materials and workmanship when installed and operated according to instructions. Should your vent require service parts, or if you have questions, please feel free to contact us by phone, letter or e-mail.
The link to the Fantastic Vent site is as follows:
Fantastic Vent Fans and Endless Breeze Fans