I had to deal with this with Wally.
You have to find that trigger to get her to bark. It could be another dog barking at her (that seemed to be what made Wally start barking) or getting her excited enough to where she just let's out a bark.
Try everything - different sounds, youtube/tv shows with dogs barking, you "barking", getting her wound up in a game, pull her tail (gently!), hold her paws for a long time (again gently).
I know what you're going through. I thought I'd NEVER get him to speak on cue, but using capturing and praising any sound he did make eventually won the day.