Balding: Does Avodart (dutasteride) work in the long term for balding/thinning hair?

I've been taking gernic Avodart called Dutas for thinning hair for two years now. Theoretically it works similar to Propecia only better as Propecia blocks one form of DHT and dutasteride blocks BOTH forms of DHT. I shedded at first as is expected to make room for healthy new hairs, the shedding stopped and my crown filled in quite well, but of course not like when I was fifteen - it was still a little thinner in the back. It stayed that way for over a year, but now, two years after starting Dutas, the shedding has begun again. The hairs that are falling out are thin and weak looking. are they ones that have been holding on all this time and are just now falling out to make room for better ones, or has the medicine stopped working and it's back to balding? Should I stop the meds or keep going? Once again there is noticable thinning in the back after 1 1/2 years of satisfactory results. Can the body build an immunity to this medicine?