Issue
A workflow with a State Machine action set inside of it never completes, and the State Machine branch loops endlessly.
Resolution
State Machine states are effectively loops, and unless the current branch is directed to move to another branch within the State Machine, or the end of the State Machine, it will continue to execute the current branch until the workflow runs out of system resources and errors.
Make sure that all State Machine branches have a Change State action that directs the State Machine to another branch, or to the end of the State Machine.