The Flash film has been in turmoil lately. After losing its first two directors, we haven't heard much of anything about it. Today, Variety is reporting that the movie is getting a "page one" rewrite and is being redone from scratch.
Joby Harold, who most recently wrote King Arthur: Legend of the Sword, has been tapped to write the script after Rick Famuyiwa left the project earlier last year. One of the reasons Famuyiwa left was allegedly over differences with the script, and Warner Bros has decided to completely start over from scratch. Since Ezra Miller is going to begin filming Fantastic Beasts and Where to Find Them 2 and not be available for The Flash, the studio is using the downtime to get the script done.
The Flash was originally supposed to release in February 2018, but with a total script rewrite there is no way that will happen. Basically, this means The Flash has been delayed due to logistics, they just haven't told us yet. The DC films have a busy schedule with Wonder Woman this June, Justice League in November followed by now only Aquaman next year. We'll have to wait and see what happens because The Batman and Justice League 2 also need solid release dates as well.