Try mini marshmallows

LOL
My SIL's chihuahua refuses to "lay down" too. But she didn't try very hard to teach her. She plays fetch much easier than my Frenchies who go and get the toy and then hog it forever and never bring it back.
I started working with Isabelle on her tricks at 8 weeks old. She knows give paw, other paw, spin, up and spin, up, down, sit, speak (which she usually just makes a strange noise but I let her cause she always seems like she's choking LOL probably just being a brat), and a few more.
Titan just knows, come here, sit, up, and now down. I started working on spin every once in a while but I needed to get "down" first

it's honestly one of the toughest!!