This is most likely due to their AWR Status flag; if the candidate has more than 12 weeks (or the amount you have set for AWR monitoring) of timesheets and their AWR Status has not yet been set to either Compliant, Exempt or Post the system will cease generating timesheets:
To set the status simply select the assignment and change the AWR Status to the appropriate setting, after which the system will continue to generate timesheets (as long as the assignment End Date is in the future).
Please note if the 'Post' AWR setting is used the assignment will still remain red flagged but timesheets will continue to be generated.
If the End Date of the assignment has also passed you will need to manually create any missing timesheets (this can be done in the Create a Candidate Timesheet option from the Timesheet menu).
The Assignment Summary report is the best way to monitor current assignments for AWR purposes as the AWR status for each assignment is shown on this report.