How small is she? If she's too small, it could have caused problems with her reproductive organs. My chihuahua was one and a half pounds and never went into her heat either. Where's Scout? She would know, she's a vet tech.
I thought it was a Chihuahua. The female Maltese I was talking about is actually one of my dog's babies. I gave it to someone. I thought maybe they just missed the heat. She did go into heat later though.