When Elka lays down, she'll frequently do it front end first. So, I watched for her to lay down, or lured her not using her "down" cue, and started giving her the "Yes!" word (this was before we were clicker training) when she was partway down (i.e., with her butt still in the air)
I clicked and praised for play bows, after about 2 weeks of that and her offering it several times per session I started saying bow as I clicked. It took about a month before she got it solid. Good luck!