My 3 yr old shihtzu will not eat from a bowl or plate, only from the floor. She is not a big eater and I give her both canned and dry food. Why won't she eat from a bowl or flat plate? She is otherwise healthy

Put her food in the bowl.. and leave it out.

When she gets hungry enough, she'll eat.

Personally, I don't like to feed into my animals qirks...

Has she always done this or did the behavior develop over time?

It could be any reason from a learned behavior from weaning and learning to eat puppy food to an uncomfortable experience where her bowl/plate kept moving as she tried to eat.

You can try to do as Lucky suggests and force her to use a bowl or you can try teaching her to eat from a proper dish. Will she eat off a placemat or chopping board?