To come... ganache is an emulsion of chocolate and liquid, usually cream. Achieving and maintaining the emulsion sometimes goes easily and sometimes requires great care.

High cacao dark chocolates require more water than normal if adapting a recipe for ordinary dark chocolates because some of the water is absorbed by the solids, making it unavailable so the emulsion will not form. Not sure if it can come from extra cream, or the fat balance means it should come directly as water.

Emulsions do not form / will break if the chocolate is too cool when mixing. It seems the presence of cocoa butter crystals interfere with or are unable to participate in the emulsion. If chocolate is tempered, it should be lightly tempered to minimize this problem. Fully melted chocolate will emulsify but then is not tempered.

One way this problem can sneak in is when using old chocolate that has form VI crystals, which may not fully melt at theoretical working temperature unless the chocolate is taken through a complete melting / cooling tempering cycle. This is one reason I often make ganache from manually tempered chocolate rather than rely on hot cream to melt the chocolate and leave it tempered. It plainly doesn't melt when the recipe says it should, leaving a problem for the type of temper it has (though that may not be a big issue) as well as for this emulsion issue.

The emulsion will break if the ganache is disturbed while it cools. At a certain point, it is safe to agitate it again, perhaps this is when a sufficient crystal matrix has formed to hold the emulsion together? This point is when it feels plasticy. The safe temperature appears to be when it is below the melting point of form IV crystals, at which point form V crystals should be well established. Still working on where this threshold is though.