When it was announced that X-Men: Dark Phoenix and New Mutants would be even further delayed, fans began to speculate on what went wrong. There were even reports that some screenings for Dark Phoenix were marred with negative reviews, but it looks like there is more to it than that.
Let's start with Dark Phoenix since it's the bigger of the two films. According to Collider, the delay for Dark Phoenix was a mixture of re-shoots and rescheduling. While the report doesn't mention the supposedly negative test screenings, it does confirm that re-shoots are part of the reason why the film was delayed.
As for the scheduling, returning stars like Jennifer Lawrence, Michael Fassbender and more have busy schedules and could not be back for the re-shoots until August or September. With only a month to finalize special effects for a November release, the decision was made to delay the film to February 2019.
NewMutants is a different story. When the initial delay was made, it was done to make the film scarier and even add a new character or two. Interestingly enough, the film is apparently not scary enough and even more re-shoots are done. The report claims that 50% of the film will be re-shot, which is surprising.
X-Men: Dark Phoenix is now coming out on February 14, 2019. New Mutants will now hit cinemas on August 2, 2019.