Going into production on a sequel to a movie or a franchise which is already a bona fide hit should be one of the few sure things in an industry in which getting anything at all off the ground is a monumental effort. That, however, is not always the case.
It might seem like a no brainer to cash in on the audience that already existed for the previous episode and keep churning out new parts until that audience gets sick of it, but there are still a load of stumbling blocks that can trip up even the most wanted sequel on its path to the big screen.
Maybe the studio has had a falling out with the director, or has lost the rights to the property, or just can't settle on a script good enough to live up to its predecessor. All of these can keep a highly anticipated sequel in the cinematic purgatory of being perpetually "in development".
And while there are success stories like Bill & Ted 3, there are still some huge sequels currently still trapped in development hell...