If you're having trouble booking a child into a session on the booking details screen, please ensure you have completed and performed the following checks.  

Check 1: Make sure the days are bookable for the session that you are updating. You can check this via Centre settings > Products > Days Bookable.

Check 2: Days bookable for the sessions are marked in circles with red rims; see an example shown below

Check 3: Ensure that you've selected the correct 'Term Time' and 'Session' for updating the product calendar. 

Once the correct term time and sessions are selected from the drop-down, the days bookable will be highlighted in dark black font. You can click and drag your cursor on the screen to make a selection or select individually to update the session.