I am unable to use voice to request start of engines. I ask for push back and engine start. The ramp agent states release parking brake, I release it, and tell ramp agent brakes are released. My next step is to ask if it is clear to start Engine 2, but I never receive a response from the ramp agent. Any suggestions?

Voice control is clearly working for you, which good.

Are you positive you're using the phrases exactly as written in the manual?

Also you ensure you "trained your voice profile".