So I find you have to kind of see what you have.
If you have thick thick grass, throw small seeds like clover and brassica, then cut grass on top of it, the thatch layer can be too thick for the small seeds.
@Crimson n' Camo says he would often mow once late summer to lessen that thatch. Then at planting time throw seed then mow again. If he didn’t do that first mowing weeks before planting, the thatch layer could just be too thick.