Rng is Rng. I got 5 cores in first mine, and then no more mines for quite some time. I am on my third now, the second had like 2 cores, and the third is the motherload, already 50 jelly, 3 cores & 2 or 3 fragments (up to 5 total now) ... and Ive only done 2 wings of it.

All I can say is that spotting these is tough. I thought that spike fences and a dwarf meant no dungeon, but circled back and discovered the mine in spite of that. There isnt a hard and fast rule that i have found -- check every black marble building. And not sure if they can be buried -- Ive even dug a little below a few of the abandoned ones I found.

I don't like the core progression thing either ... its a repeat of the forest and you either find a bunch or none at all in a dungeon, with the dungeons being much rarer. At least most forest have them every 5 min walk or so.