What happened was that the writers and production people in charge stopped caring about the product. There was tension between the creatives and the company and the creatives started calling it in. The last few seasons were a half hearted effort--not just on Flash but on Batwoman and Supergirl as well. This was felt all the way down to the stunt people and filming crew. I was told this by a fight coordinator who worked on several of the series and was a stunt person/stand in (don't know the term) on Flash, Supergirl and Batwoman --that the attention to detail and caring about the final product just changed in the final couple of years of production. When the people running the show on the ground stop caring--that feeling runs through the entire production.
