Even though I have enough medical evidence to prove my disability, Lincoln Financial keeps denying my claim.

I was diagnosed with Fibromyalgia in 04/2012. I was out of work for quite some time, which I exhausted my FMLA hours. During my my absence I applied for STD with Lincoln Financial and was paid. I was lucky and did not have any flare ups for a long time. Then in December 2013, I experienced a really bad flare up and was out of work, applied for STD, was paid. Unfortunately, I had another flare that happened shortly after I returned to work, causing me to be out of work for quite a while. I applied for STD, and was denied. Filed the appeal process and was denied again. I was told that there was not enough medical evidence supporting I needed to miss work. It was exhausting to keep fighting with these people. However, it confused me that the same company approved my FMLA, where the Dr. states due to my medical condition I may miss 12-15 days per month. So there was enough medical documentation that I can miss work but just no get paid for it I guess. So here I am again terrible flare causing me to miss work and being denied for the same reasons. Please help, it is bad enough to have to go through the pain but now not to get paid for something I am entitled too. Sincerely, Kim