Exclusivity is the main issue between the two sides as WWE wants complete rights to Ric Flair while he’s under contract and Flair still wants to be able to work independently. This was the main problem they had that caused Flair to leave the last time.
Additionally, Flair has publicly stated that he is not against the idea of wrestling again, but WWE apparently has no plans for that.
After WrestleMania 24 and Flair’s "retirement", he was reportedly making $500,000 a year for his "goodwill ambassador" role with the company. However, take the rumored $20k a month for alimony, the huge chunk for taxes, and the way Ric Flair is used to living (excessively, carefree, etc.), it’s understandable how he could believe that wasn’t enough.
He also passed on working as the General Manager of Smackdown! that he was offered during WrestleMania 25 due it not paying enough (rumored to be $250,000/year).
At this point, it is being said that Flair is happy to keep working selective dates for the WWE without being under contract, allowing himself to stay working independently as well.