A timesheet for a candidate which is 'Awaiting Approval' but the manager is saying that it shows up as no longer valid?

If the timesheet has a status of Awaiting Approval it means it has already been received back into the system, therefore it must have been signed already and is waiting to be validated in either Incoming Images (if faxed or uploaded) or Validation (if electronically signed).

We would recommend you check these areas for the timesheet, plus Timesheet Display is a good place to start if you want to find out what status a particular timesheet is currently.