If Feature B isn't ready, it should stay in its own branch until it is.

Develop is for code that the developers say is ready. You might have bugs, poor merge resolution, etc, but any fixes made should be quick and should pave the path toward code that can be merged into master. If the problems are major, revert develop.

Master, on the other hand, should always be stable and rock solid. You can then have production servers that always pull master automatically, and staging servers that always pull develop automatically.

I've worked on multiple teams this way it works out quite well.