Have the timers all been FM-0c and FM Hornet timers? Is "F2B" a brand of ESC, and if not, what brand of F2B ESC is it?
I know that some of the Hubin timers are designed to work with a plain-old ESC without governing, and increase the throttle signal to the motor over the course of the flight, to compensate for battery sag. I don't know which timer is which, though.
I would assume that an ESC that's marketed as "F2B" would have a governer, such an ESC already does the governing, so a timer that tweaks the throttle signal up during the flight would result in higher RPMs as the flight progresses.