Mammillaria seeds can germinate very unevenly. Some species of Mammillaria germinate better when the seeds are a bit older. Mammillaria luethyi, Mammillaria goldii and Mammillaria theresae seeds for example have some kind of germination barrier which needs to be broken down before the seed can germinate. There is still no clear proven method to germinate these seeds. The best thing to try is to sow them like other seeds. If they do not germinate in the first attempt then let the seeds and germination mix dry out for a couple of days and then try again and again. The seed coats eventually get worn out and they start to germinate. Still, these seeds present a real challenge but you get rewarded with a beautiful plant when you succeed.