Another reason might be because you have already set another lesson at the same Venue, Room and Time. The system will not allow you to choose this time slot and place again. You can read how to change Lesson details here.
Checking opening and closing times of a venue
Available time slots for any lesson start at the opening time set for the chosen venue. Opening and closing times are set in ‘Venue Detail’. To check them, go to 'Settings' > 'Venues' > 'View all' and click on the venue you need.
You can check the opening and closing times in ‘Contact details’:
Changing opening and closing times of a venue
If you would like to change times when a venue is available, press the ‘Edit’ button in the top right corner:
You can now choose any other time interval for your venue:
Make sure to save any changes by clicking on the ‘Save’ button in the right corner:
The changes are now successfully applied. Now you will be able to choose timeframes for your lessons within the interval you have set.