At Last!!!

Mammillaria hernandezii has been a plant that I have found great difficulty in getting into flower. they always budded up, but just sat there all winter and aborted in the Spring, sometimes causing the plant to rot.

This year, I saw that flower buds were developing so brought my two plants into my house, kept them warm, and gave them some water. A few buds have aborted, as perhaps they didn't get the best light, but some are fine and the photo below is the best of them so far. I'm now wondering whether once flowering is over, I should continue to give them a little water or just let them rest.

Congratulations on the flowering!
I have exactly the same experience, the buds do not develop into flowers and are then aborted in the spring. But if I put the plants in the warm house, the buds develop and some of the plants come into flower. However, I have not been able to obtain fruits after pollination in that case. Apparently, it was not warm enough in my house to get healthy pollen. What did happen a couple of years ago, is that the decaying flowers caused a few plants to rot. So, you might want to keep the plants in a dry place in your house until the flowers have dried up completely.
Another thing I did a couple of years ago was to put the hernandeziis in a darker place in my greenhouse (from early August on). In that way, buds did form but stayed quite small and some of them survived until spring and then flowered. I could successfully pollinate these flowers and got fruits with very viable seeds (and many seedlings afterwards). This year I am trying that method once again for several hernandeziis, although I have put them under the shelves later than the first time. A few others were put high in the greenhouse and these have much larger buds than those placed rather dark under the shelves. I expect, however, that these high-placed hernandeziis will not come into flower and that the buds will probably shrivel in early spring.

Wolter ten Hoeve.