I was given the quest to collect color from the mines. OK. Got both glyphs, and the mine in Sister Death's chambers was no problem. But the other mine... Not only do my expose glyphs only register about 10% of the time, but even when the harpoon hit the colored area right in the center, nothing happened! In addition, I was only able to locate 2 of the 3 mining nodes, and the two I found were extremely high up, which I guess is why they were impossible to mine.

Of course, Mantid decided I was a liar and killed me.

The harpoons can be rather tricky at first, since they are affected by gravity and will not land exactly where you mark them. Also note that you can cast the glyphs from anywhere, so if the glyph is too high on the wall try getting as close to "eye-level" with it. You don't necessarily need to be under the vein for the glyphs to work.
In Ava's mine, I would recommend climbing the tower to get a better shot. This can be achieved by taking the left path all the way around from the entrance.

Be sure you're drawing the glyph's large enough, and that you actually have enough color to draw the glyph. Although the exact number depends on your "equipped" colors, I think it's a minimum of about 16 per casting.

Where the veins are generated is entirely random, so I would recommend loading from a previous save and trying your luck again.

You can wall climb out of the map on the mines dude, it's not broken just climbety climb your way uppety up

I Love the jumping bug/glitch/intended feature gets you around those pesky predators!

Thank you very much for your tips, everyone. I have perfected my expose glyph (I wasn't using enough color). I scaled the walls, climbed the tower, and hit every node several times right in the center, but still they don't break. I suppose my save is glitched? I guess I'll start a new game... I was actually doing pretty good that run. Sigh...

Am I correct in assuming the mining nodes are generated when you first enter a chamber? I suppose on my new game, I'll make a save before I enter and just reset until I get some that work correctly.

That's so weird that it works in one mine but not the other.

The veins are generated at the beginning of each cycle. So you could actually load up your save from the previous cycle as opposed to starting over.

Well, I did start a new game, and I got to the point with expose and... now it won't work in Sister Death's mine either! The harpoon fires, and bounces uselessly off the wall.

I really wanted to play this game...

If you get a harpoon, you're using extract, which is the one you have to do second.