Four bushings hold the planting unit to the toolbar. They wear fast, and worn bushings cause the planter to nose down. Gauge wheel arms. If these are worn, you're likely to see mud inside the hubcaps. In wet ground, a quarter-inch gap lets mud into the hubcap.
Dry dirt is worst.
With the gap, dry soil flows though the opening right onto the newly planted seed. You rob the seed of the moisture that otherwise might be available in the seed trench. If the closing wheel is off center, Sauder says, you may be running the wheel right over the seed trench instead of along the sides. One way to check is to put the planter on a piece of concrete. Lower the opener lightly onto the concrete, and pull the planter ahead a couple of feet.