Microparticle performance rating might be too high, trapping smaller particles but not suited to a higher air flow. I would try a less restrictive filter.
Trying to second guess a tech that was on-site can be difficult without any other information. How many return air vents did the home have? Was the bonus room over the garage present when the original system was sized and installed? Or was it added after the fact? What geographic area are you located in?