Run the plow back and forth fully 3 times on the ground, raise it, run it back and forth 3 times, down, then check the fluid & let sit for 3-5 minutes, repeat, do this 3 times. See if there's a difference afterwards.
If not - check the adjustment on the cross over relief valves using the guides below. (Assuming it's a SEHP, if it's not and you need the diagrams, just let me know.).
If that doesn't work; flush, drain, & clean it, pull, clean & inspect the 3 valves, paying close attention to the S2 valve. Replace with good fluid and run it again. If it hasn't been run in awhile, it would be my first step regardless.