I was wondering if maybe she feels trapped and helpless being tied to the cart? Maybe she feels defenseless if something should happen. I did find that she looked uneasy with the whole thing. Maybe take the above advice and go about your business as if everything is fine and dandy. And if she does take a step, lots of praise and a special treat that she gets only when attached to the cart and does well. If she knows the recall maybe try calling her for a treat. If she won't come for that, place a treat 1 foot away from her so she will have to go forward to reach it....something she loves and with lots of good smell to it.
Give her a bit of time yet and if she doesn't come around maybe speak with your vet.