How do Echoes work? (Mild Spoilers)

[darugo06]

So, I've found where most, if not all, the echoes are located, but two of them seem to not be appearing. I got to the area and triggered a sort of flash, where the lightning got all weird for a moment, but the cycle after I wasn't able to get to the spots because the areas kept killing me in stupid ways and now I can't seem to trigger it again.

Did I miss it permanently or is there some arbitrary unspoken rule to triggering them?

[BirdoPrey]

Some gods are harder to perceive than others. After the blue flash, you must hibernate and then return to the area where you saw the flash. There is also a karma requirement, though I'm not sure how it works yet, I think if you're near the top of your current karma-meter it should work.