I think it depends on how you calibrated the monitor in the first place.  If the accuracy does appear to "drift", put some actual weights of the loads that are off into the monitor and see if that works.

Very rarely will the load cell in the flow sensor give this problem...it either works or it doesn't in most cases.  It more often has to do with how you calibrated the monitor, what kinds of loads you've put in thus far, how tight in the clearance between the paddles and the deflector plate, etc.