I’ve had luck with the VFW, but YMMV depending on where you live. Make sure to file your Notice of Disagreement within a year of denial to preserve the date of denial. The package they mail you will explain why they denied you. Once you file your NOD, it could possibly take years to go through the appeal process, however there’s a new program called RAMP that has been speeding up their claims, usually in favor of the veteran.