There are two reasons why the shift you were invited to is unavailable:
1. Accepting the shift may lead you to be working too many consecutive hours
If accepting a shift invite would put you over 18.5 consecutive hours of confirmed shifts, you will be prevented from accepting that shift.
2. Accepting the shift would overlap with another confirmed shift
A shift to which you are invited may be unavailable if that shift overlaps with an existing confirmed shift.
If the shift is unavailable due to one of these reasons, it will be marked accordingly in the "INVITED" tab.