What did Bartle spend a great deal of time trying to identify when he noticed changes in Murph in Chapter 8?

Bartle spent a lot of time trying to identify the exact point at which he had noticed a change in Murph. He thought if he could figure out when Murph had begun to slide maybe he could do something about it. He also wanted to find the cause of Murph's slide and trace it back to something that he was not responsible for causing.