Mocha's left ear was also flopping down, and she was 16 weeks by the time I got her. It flopped after I left the breeder, called them up, she told me to tape it, but as we know Yorkies aren't the most submissive dogs
So I tried trimming it.. It flopped once more when the hair started growing long again, but one more trim and I think it's done now.
You can see in my avatar her chest hair have grown kinda long but the ears are still just.. ears lol