Ensure if you are selecting all employees from the Model Input Payments listing that you expand the display dropdown to show all employees then tick the select all box.
The employees who have a fixed end date added to their basic pay record, or those who already have a pay element with the same name as the one you want to add through the pay modelling within the same period, will not appear in the listing when applying the pay model. This is by design.
To follow the correct process, follow these steps.
Open the employee's most recent Basic Pay History record.
Remove the Fixed End Date.
Save the record.
Return to the listing stage of the pay model workflow from your task list.
Finish Pay Modelling.
Return to the Employee's Basic Pay records.
Re-enter the Fixed End Date on the new basic pay record.
For those who already have the pay element you want to add within the same period, you will need to create a new pay element (e.g. Basic Pay1 for example instead of Basic Pay) so that you can use it for this pay model.
This way, the missing employees can appear in the list.