Roxy does that too. If it's raining she won't leave the porch. Her ears go back and she shrinks up like the rain is beating her.
I just suck it up and go out with her as opposed to just telling her from the porch to go. She'll tip toe around, (She doesn't like her feet to get wet either, *sigh* what a princess she is) and go as quick as she can.
If they still won't go I'd put a leash on them. Maybe lure them with some treats.