How to increase my 1 yr old vocabulary

My daughter just turned 1. At the 1 yr checkup the dr was surprised that she only says mama and dada (she said dada for a week than stopped and now everything is mama). The dr wants her vocabulary to increase for the next 15 month visit. She told us to illustrate what we do to my daughter. The thing is we have been doing that. I also spend about 40 minutes daily reading to her. She understands allot like up and down, turn the page, etc. What else can I do to.help with her vocabulary?