When your Medicare medical insurance denies to pay for a service already provided by a medical practitioner, by law, you have the right to appeal the denial. Unfortunately, One Exchange aka Towers Watson will not help you as they have no legal requirement to help you resolve the situation nor have they contracted with IBM to provide such services. As I have written so many times before, they are just insurance agents. You are on your own. Any complaint you decide to pursue must be done via the the standard Medicare appeals process. It is a legally defined process and the process steps depend on whether you are using original Medicare or a Medicare Advantage plan.
It's almost always worth it to appeal an insurance denial. There are some obvious "not worth it" situations such as using a doctor who does not take Medicare or having a doctor preform a procedure not covered by Medicare (such as Lasik cataract surgery) and you signed an ABN (advance beneficiary notice) telling you Medicare might not cover it. However, the denial is more often associated clerical mistakes like a provider coding a procedure incorrectly. There are always instructions in the insurance denial letter on how to do an appeal. The instructions are reasonably good. Also, don't give up if you are denied a second time. Appeal again! The reversal rate is something like 50% for people who are persistent about appeals.
I learned something today that is important to know if you decide to do an appeal. Your provider cannot hound you for full payment of a procedure until all the levels of the insurance appeal are complete. That can be a lengthy process. As soon as you decide to appeal, call the provider billing department and follow up with a written letter telling them you are appealing the denial and not to bill you. They, then, (by Medicare law) can only badger you for the copay or coinsurance for the procedure until you complete the appeals process. If they threaten to send your full bill to collections, tell them it is illegal and you will file a complaint with Medicare if they do it.