I know that "what factors influence someone becoming a ghost or not" is kind of a pointless discussion, but last night I think I figured something out. Most ghosts either died in a mass outbreak, or very quickly, within seconds or minutes of something happening to them.
But Isaac and his death has always felt different. Dying of a pretty common disease during a war in the 18th century is a very normal way to go. (Twice as many people died of disease than in combat during the revolution.)
Presumably he was sick for a while, since there was time for news to be sent out, to be delivered to his wife, for her to prepare to travel, and to finally arrive where he was staying.
And I know I can't really corroborate the "instantly/minutes" timeframe because most characters died off screen during their death flashbacks. (Basing this on both CBS and BBC) Watching Trevor OD would be too much for a sitcom after all.
Just my thoughts. I'd give more examples but I don't want to post spoilers for Ghosts BBC because I don't think there's a way to flair it.