M.thresae pollination

For 2-3 years, I was trying to get seeds cross-pollinating some of my Mammillaria plants, but yet no luck. Namely, it was M.thresae (one old plant and one young seedling) and M.perezdelarosae (two grown seedlings). Definitely there were tens of flowers every year, but whatever I did was fruitless. Any ideas or hints will be highly appreciated. Thanks.

I don't really know what permits cross fertilisation, but I thought that the parents had to be fairly close in genetic terms. The two you have chosen are from different Series, and from what I remember about those DNA studies that have been done suggest that Mammillaria as we know it today might actually be polyphletic, which in practical terms means that those species might be really in different genera.
I know there are some intergeneric hybrids, such as Ferobergia, but although the appearance is quite different, in fact they are quite close genetically.

Sorry if I wasn't clear enough - I was trying to pollinate two theresae plants to get theresae seeds and two perezdelarosae plants separately to get the perezdelarosae seeds. It wasn't the cross-pollination of theresae with perezdelarosae. My apology for the confusion.
Unfortunately, these are one of my valuable plants from which I couldn't get any seeds for years. Never had such a problem with any other cacti.

Oh, sorry, my rant was not relevant. I shall remove it.
M. theresae tends to keep its seeds within the plant body and rarely develops an external seed pod as most of the others. Sometimes if the plants are actually from the same clone they may not fertilise. Unless you know the source of your plants, it is impossible to know.

Both of my perezdelarosae plants are seedlings, but from the same package so they might be genetically close (siblings). One of my theresae plants is an offspring I got many years ago, but the second plant is a seedling I grew myself. So, there shouldn't be any issue I think.