If you've checked the client to staff ratios are ok and the staff member isn't doubled booked here is another reason the shift might display in red. This article takes you through how to troubleshoot this.
If a staff member had annual leave, a non working day or was previously booked on another shift when another shift is scheduled then they later cancelled that leave or were removed from the shift you will see both the new shift and the canceled leave in red.
When hovering over the shift you might see the staff in green indicating there isn't a problem with the shift.
The reason for the summary view showing the shift in red is the system is remembering the availability of the staff when the shift was created.
The simple fix for this is to edit the shift or program (if a series of repeating shifts is affected) then re-save the shift to update the status.
Once saved the summary will show the shift in green and the cancelled leave in red as expected.