I find this interesting as well efors. When I first got my pair, they bred like mad every 4 weeks or so. I'd see a few come out of the shells first, and then eventually they all would appear. I've counted broods of 15 and 22 and not all would survive. Now, every once in a while, I'll see 2 or 3, and then nothing. I have two theories, diet and/or intruders nabbing them. I've got the original pair, plus 7 other offspring of theirs, and it's like they've decided that they're the "chosen ones" and no others will survive. They are in a 20g. I had been feeding them Golden Pearls, and I recently started alternating that with NLS flake to see if it changes anything. Perhaps a higher protein snack like bloodworms could induce it too. Female conditioning plays a big part in what she can produce from what I understand.
So maybe a diet change or water change with a temperature change will induce better results. Just some ideas to ponder before you give up.