What is a word or phrase for when an artist cannot live up to his (or another author's) earlier work?

You can say an artist is past his prime.

An unkind way to put this is to refer to that person as a "has been"; this means the person was once something special but no longer is. A kinder way is to say that the artist has "peaked" (or specify a high point, e.g., "Mike Nichols peaked with The Graduate").

As noted by yourself, "measure up" is the phrase which defines when an artist does not create a work at the level of his previous work.

Example:

"Quentin's latest work did not measure up to his previous: The Antique"

Other related expressions are: "One-hit wonder" - a performer or act, usually a singer or band, who has just one popular success. "One-trick pony" - a person having a single accomplishment.

A more recent largely Internet based colloquial expression would be "He's jumped the shark". (Etymology: The sitcom Happy Days is largely regarded as having lost most or all of it's meritorious quality by the episode in which the Fonz waterskis over a shark.)

You could also say that the artist is "overshadowed by his previous work".