To check for any gaps in register please go to Reporting > Reports > Attendance (tab) > Gaps in Register by Date:
In this report if you are greeted with the below message you will need to go to Admin > Config > Core Set Up > Holidays to ensure that none of your dates are overlapping:
For further information on removing the overlapping dates please click here.
Once in this report ensure that in the top right you have clicked to show gaps.
Any dates where codes are missing will be circled in red, if you click into the circle you will then have the option to fill in the gaps.
If you click onto fix gaps this will take you to the manage register area where you can click into the red box to add the required code.