Cactus / dasyacantha

Some of Mammillaria are difficult to germinate from seeds.
I bought seeds of hernandezii, dasyacantha and saboae and sowed in 2006. Dasyacnatha was only germinated but it was also very low germination ratio.
I tried hernandezii and saboae several times later but they never germinated.
I grafted 2 seedlings of lauii v dasyacantha. One of them is in flower currently.