You may have accidentally marked that a FreeFlexer should be blocked for your future shifts. During the checkout process for a FreeFlexer you have the option to disable this person from applying to your shifts again.
How can you give someone access to your shifts again?
Go to the menu by clicking on 'Hi, [your name]'.
Click on 'Blocked FreeFlexers'.
Here you can see who you have excluded, even for multiple locations. Click 'Unblock' on the freelancer you want to give access to your shifts again.