I hurt my hamstring 9 mos ago doing yoga and took a while off but still can't go back, what's going on?

Healing with scar... It is not uncommon for hamstring and other muscle injuries to lead to chronic problems. Muscle injuries initially heal with scar tissue. Without proper stretching and strengthening rehab exercises the injury can heal in a stiffened and weakened state and lead to chronic stiffness and risk for re-injury. You should see a doctor for further evaluation to rule out more serious injury and guide rehab.
Chronic strain. These can linger for 9-12 months. Make sure that you haven't pulled the hamstring off the bone. An xray or MRI may be necessary. You may need physical therapy also.
Calcium buildup. When a muscle becomes injured the body will setup a defense mechanism and lay down calcium in the injured area. Go see your md and he can set you up with a physical therapist to return your hamstring back to normal function asap.