My gelding had the same exact thing happening when I first got him! I found out later though, that I was the one causing him to drop his shoulder. I finally had a trainer out watching me and she noticed that I twisted my body to the outside instead of staying straight with him. So since I was twisting, he was dropping his shoulder and over compensating, which caused me to try and over correct. We were a mess for a while.. haha! So perhaps you could have someone come watch and make sure it's not you? Not saying it is, of course.
If it's really just a lazy horse, I would just suggest LOTS of inside leg and making her bend her body. Small circles until she picks up her shoulder and supports herself. Then move out to bigger circles.
Hope that helped! :)