Teaching The Speak Command

I’ve had great luck teaching two previous dogs to Speak on command. When they were puppies and barked at anything, I would introduce the word “Speak” with a good mix of praise and some treats. One of the two even learned “Sing” and would howl on that command.

I have a great dog now, but she does not seem to associate barking with the word no matter what I do. Could it be her age, six years old? Or breed, some kind of mix with maybe Bichon? She is very attached to me and smart, she obeys commands and hikes off leash with minimal training.

Any help how to help her learn Speak?