Ava is a woo wooing machine. She always has been, and we have encouraged her by fussing over her every time she does it. There is a picture of Ava in our gallery in the midst of her littermates woo wooing at just a few weeks old.
Her older, half brother/cousin Indy never woo wooed, but now, will occasionally mimic Ava, especially when she is doing it, and he wants attention. I wish he'd do it more often, he has a much sweeter voice than Ava. Hers is good, but his is outstanding .......... when he decides to use it.
As you have noted, they usually do it when they want something. It may be food, or attention.
You can try woo wooing at her. Ava usually responds to that. When she does, make sure Nala knows that you appreciate it by fawning over her.
Ava will do it with pretty good reliability on the command, "Sing".
You can try to coax it out of her when you are playing. Just back off, look at her, but don't give her any attention. It might prompt her to woo woo at you.
Be sure to praise and/or treat her at the slightest sign of a woo woo, and procede from there.
Let us know when she begins "singing" again.
Tom