The paint is on there now and there is nothing to do but touch it up when ever it chips. It's possible there was some dust or something between the paint and primer on the spot you're having trouble with. Before painting something else I would research the compatability of the paint you used with the primer. Sometimes a person can use rattle can primer which is may be a lacquer primer and topcoat it with enamel and it's not compatable. The lacquer primer needs a paint with solvents strong enough to re-wet it for good adhesion.
Like I said before I would not use a power washer on the tractor. If there is a adhesion problem with the topcoat a power washer will peal the paint off in sheets.