AD7997 - No ACK after LSB of configuration

Hello, I'm communicating with my ADC AD7997 using an Arduino Fio. I'm not getting an ACK after a write to the the LSB of the configuration register, as seen in the attached picture. As a result, my ADC is not getting configured properly (as could be seen from a subsequent read from the configuration register). All my other messages seem to be being sent fine. I'm using external 1k5 pull-ups on my board, which sits on top of the Fio (no long wires). Not sure what to try at this point...