There is usually a knob that controls the rate of drop for an implement, that can be turned all the way and not allow an implement to drop. It’s usually down between your feet below the seat.
See if the tractor has a flow rate adjuster under the seat or on the rear someplace. A few months back I was getting bounced around while doing some dirt work and I kept cranking the seat adjustment. Suddenly my gannon would not go down, then I realized I had been cranking the draft control, not the seat adjustment while operating.