Over packed soil wicks to much water, leaving no airspace in the soil. The result is inhibited or slowed germination. If trays are filled too lightly, the soil will not wick enough water for germination. Compacted soil is the main cause of the spiral root problems. Spiral roots can occur when the seed germinates and the root cannot penetrate the soil mix because of the lack of oxygen in the soil.
Roots grow in a circle or spiral on the surface.
Dibbling devices should provide a consistent depression for seed placement. We prefer the pyramid over the round type dibble. Make sure the dibble board is constructed to fit the tray that you use. Trays of same cell numbers but different manufacturers have slightly different alignments. A mismatch can cause seeds to miss the dibble mark which could cause poor germination.