HMO hired intermediary that denies care. What are options?

I injured my back 15 yrs. ago and have dealt with it myself over the years. Earlier this year, it bacame unmanageable, so I started seeing a chiropractor which manages the pain enough to keep me mobile and working. My HMO just hired an intermediary company that seems mostly to deny treatment. I have been denied any more treatments, so I've missed more work....more than I've ever missed in the twenty years I've worked at my company. I sent an appeal to the med. ins. company, but don't have much hope. It seems senseless to deny treatment that keeps me upright and working at my desk versus the cost of back adjustments, and this isn't even taking into consideration my time off which is now staying home, rotating between the couch and the recliner. I'm off again this past week and probably in big trouble at work for missing all this time. I pay for med coverage, why would they deny something that is working?