This may be due to timesheet group admin permissions.
To check this, you can go to Timesheets > Timesheet Approvals and select Timesheet Administration.
From this screen you select the blue hyperlink "1" and a pop up will appear for all user permissions.
On this screen you will be able to view the status of permissions in the "Timesheet authority" column.
If this column is set to "None" the user will not have access.
You can adjust this column accordingly - with multiple entries possible for a single user.
Entries are added using the "New Lines" button.