There are many... frequently the sum of the parts don’t add up.
Lack of shooting, an obvious defensive weak link, or too many guys that need th ball are the culprits.
Minny, OKC
Honestly the Heat teams were way more talented than their performance... things didn’t fit perfectly but those were super duper stars and they were elite defensively.
Lack of shooting/space murders offenses.
Lakers adding Malone and Payton made them a lot worse. Pippen joining Barkley and Hakeem did not work out. The Nets super team a few years ago did not work out.
Ron Mexico is right, I think, in that people overestimate chemistry a bit. But like when he says "height doesn't matter, only reach does", he's taken a good idea too far.