GRRM himself said that was his holdup. You can say that he’s not being truthful, but that’s a different take entirely.
An interviewer asks:
Now that we know how the “Meereenese knot” played out, what was the problem with this? For example, was it the order in which Dany met various characters, or who, when, and how someone would try to take the dragons?
Now I can explain things. It was a confluence of many, many factors: lets start with the offer from Xaro to give Dany ships, the refusal of which then leads to Qarth’s declaration of war. Then there’s the marriage of Daenerys to pacify the city. Then there’s the arrival of the Yunkish army at the gates of Meereen, there’s the order of arrival of various people going her way (Tyrion, Quentyn, Victarion, Aegon, Marwyn, etc.), and then there’s Daario, this dangerous sellsword and the question of whether Dany really wants him or not, there’s hte plague, there’s Drogon’s return to Meereen…
All of these things were balls I had thrown up into the air, and they’re all linked and chronologically entwined. The return of Drogon to the city was something I explored as happening at different times. For example, I wrote three different versions of Quentyn’s arrival at Meereen: one where he arrived long before Dany’s marriage, one where he arrived much later, and one where he arrived just the day before the marriage (which is how it ended up being in the novel). And I had to write all three versions to be able to compare and see how these different arrival points affected the stories of the other characters. Including the story of a character who actually hasn’t arrived yet.