I've installed a couple of these things. I think locally, anything over a 8' run (or around there) requires a booster fan. I'm assuming this requirement is to reduce the possibility of a back-up resulting in a lint fire.
They work on a pressure/air-flow switch that senses when the dryer starts pumping out air from the dryer exhaust.
If their fan keeps running, it could be three things: A) the switches are going bad B) the device is installed improperly (backwards - the orientation of the switch & it's little feeder tube in relation to the fan/dryer is important) or C) The GC installed cheapo crap just to knock the job out, and the units are failing for any number of reasons.