What's wrong with my dog?

She's been walking like this (hop on left hind leg) for over 4 months now. Vet has examined her 3 times and found nothing in her paw, no sign of patella luxation. She's had an x-ray of the hip and dysplasia has been ruled out. We did 8 weeks rest prescribed by the vet but no different after. During that time we've put her on Yumove but not seen any change. The vet has basically said no idea, it's beyond us and referred to a specialist.

From my lay (lack of) knowledge, I don't think a broken toe or ligament damage would have been ruled out by any of the examinations. We're seeing the specialist vet on Friday, but any tests they recommend are going to be super expensive.