Go to Tools - Salary tools - Add a heading to a period of dates :

If, for example, an insurance corresponding to item 5600 has been omitted :

Choose the item, the period and the concerned employees.

Click on execute.

Il will be created as an empty line in the closed months.

He will make up in the current month what he has to make up: 

If this is a code-related insurance, arrange to have them added for the missing months. (See details)