I'm not sure, but there's an odd bug where Deekin won't be summonable in Chapter Three if he didn't leave the party at least once in Chapter Two (either by death or by dumping).
Yeah, I've heard about this, which is why I was specifically referring to replacing Deekin with one of the others. I figured it was the most likely situation in which it may be possible to get the Reaper to summon all three henchmen.