You start to notice the effort.
Rappers do some very difficult, intricate, and artful things, but when they're at their best, it all seems effortless. Rap isn't just a technical performance, it is also theater, a place where the artist's character is inseparable from his style, his technique, his appearance, and his art. We see these as all of-a-piece, one and the same. Ludacris isn't Christopher Bridges, Ludacris is a package, from punchlines to persona to the blunt under his hat by his left ear in the "Welcome to Atlanta" video.
When rappers fall off, though, the seams start to show. If the persona starts to feel forced, like the rapper is trying to fulfill the audience's past idea of what made them great. Sometimes, they'll fixate on certain points of their persona, missing out that there was a full package that made them great in the first place. The more they try to embody this idea they have of what they are, the more obvious it is that they really are trying. Great rap makes the difficult look easy, weak rap is when the difficult seems difficult.