Harry Potter may be to blame.
Ezra Miller was one of the best parts of the Justice League film. The film may have been a bomb for Warner Bros., but Miller played his role to perfection. The comic relief and sheepish intelligence of Miller's Barry Allen was refreshing alongside the brooding Bruce Wayne, or the machismo-driven Aquaman. Ezra is set to star in a standalone The Flash film, but the ill-fated movie has hit another snag in production.
According to Varitey, The Flash was supposed to begin filming in March but production has been pushed back due to an unfinished script. The studio does not believe that they can finish the script by March, and Miller must begin shooting for the third installment of Fantastic Beast in July. The film, based in the Harry Potter universe, features Miller as Credence, who is an Obscurial. His role in the Harry Potter universe appears to be a big one, as eagle-eyed fans were able to discern from a trailer that Credence is actually a Lestrange, and possibly the father or grandfather of Bellatrix.
Since Miller will be preoccupied fighting with wizards, filming for The Flash may not begin until late 2019. In turn, the film would not be likely to appear in theaters until 2021, although an official release date has yet to be released.