Failures do happen for a variety of reasons, like bad guiding, passing clouds, weather turning bad, high winds. But also some more stupid stuff like failed slews and the likes.
In all cases, you might see at some point that some parts of an imaging request are marked as failed.
There are two solutions to fix this:
Option 1: Re-enable them manually via the dashboard. Simply click on the small button for resubmission of failed plans:
Option 2: Under User Settings, enable the automatic rescheduling of failed plans: