I agree with the above poster... Repetion has been key in my expirience. Use the same words or phrases, at the same time of day, in the same tone & eventually if they WANT to talk, they'll get it right. Birds are like kids in the respect that when they first begin speaking the pronunciation wont be perfect. Clear, precise vocal imitation only comes to them with practice & patient persistent owners! Good luck! Sounds like you are doing everything right to me.
Shadow Moon
Nevermore
Jax