How is the snow plow hooked in to the hydraulics?
I would have two guesses to look at. One, there is a 3pt lift/drop speed control knob under the seat on a lot of the "B" series. Adjusting it might help. There is also a switch/valve on the hydraulic connect. On my B8200, when using the loader, the little valve had to be turned on for proper operation of forward hydraulics. On mine, it is on the front right side of the tranny; apparently all hydraulic flow for auxillary hydraulics(loader, 3pt, ect) is directed through this mechanism.
Also, are the hydro fluid, strainer, filter ect clean , full, and good? How long since this has been changed?