Continuity mistake: After Richard cuts his hair, it goes back to its original length at least three times during the movie. (00:30:10 - 01:24:10)
Character mistake: According to her biography found in the library, Elise was born November 11, 1885, but William Robinson tells Richard that he met Elise when she was 16 years old, in March, 1903, at which time she would have been 17.
Answer: He was no longer able to put everything out of his mind, which was a requirement to successfully time-travel. He was distraught and unable to focus enough mentally. He stopped eating, and as time went on he became weaker and weaker.