I would be speaking to the vet again and let them know that you are back in the same position as last year and the increase in sleep hours has not solved the problem.
Have you completely rearranged the cage, moved the cage to a new location and made sure there are no hiding spots in the cage or anywhere else she has access to?
They generally wont want to lay and get all ready for babies if they do not have conditions right so making them a little uncomfortable by moving them and changing up their home is often a big help too.