could be one of three things.  

  does it have in-cab downforce control?    if so, the most likely culprit is the air valve for the downforce.  it could be, A: stuck it's a pilot operated air valve, and the spool could be corroded, gummed up. or, B: internal seals are shot, in that case you need a new valve.

one of the other things is a leak somewhere in the air system, depending on what year planter, there are some updates to a better down force distribution system.    also if you have in cab downforce, there are a couple of sensors on the valve that can pop their internals ever-so-slightly out, and will leak air.

lastly the compressor high pressure switch could have gone bad, it should be cutting out at about 105 psi.   

that should cover the downforce system, I would isolate the true count from the downforce and try to see where the leak is, would make diagnostics a lot easier.