It's always good to have her checked out by a vet if this is a new behavior. Your dog may have dental problems or may not be feeling too well. On the other hand, many dogs, once they have tasted canned food, don't want to go back to eating dry or may refuse to eat dry unless it's topped with some canned food just because they like canned food too much! To put ourselves in their place, it's like getting a sundae with chocolate sauce and a cherry for weeks and then, one day, out of the blue, the chocolate sauce and cherry go missing. Many of us would be very upset in such a case and will likely complain to the server. If the server tops our sundae with our favorite toppings readily upon complaining, granted, we will complain more in the future if the same mishap happens again. Your dog may have gotten so used to the canned food topping that now she refuses it when it's missing, and waits for you to add it. In such as case, she has trained you well!