Are you planting the seeds in dry soil and then watering?

Water the soil first to have it settle and then try planting. The germination doesn't seem to be your problem if you're able to sprout a tap root. Just seems like you're having trouble finding the right balance in the soil.

My suggestion?

Water the soil so that it is damp and compacts on itself before you plant the seed. You don't need to pack it with your hand.. gravity and the weight of the water is ideal

You could even do this a day ahead to ensure the top part of the soil isn't to wet . You don't need mud.. just settled, damp and mildly compact soil.

hope this helps.
Bob.