[mturmon]

I think it should be said that the right strategy is very case-by-case.

Some people will know why things are not working out, and for them, you don't need to say anything about cause. It would, for example, just re-open a wound.

Others will press to know, and it's best to have something short and clear to say, say that thing, and then elaborate no more. You can't get in an argument or go into the fact-finding rabbit hole about it (the outcome is settled).

If you don't have a reason, you will look like a fool, and this will tend to lead to a bad outcome.