1) Sometimes by the time you check the substitute website for a job, the job listing could already be gone because the job has been accepted by someone else. Especially in the more competitive districts, some jobs will be accepted in a matter of seconds and no longer available for viewing.

2) The teacher or school district could have canceled or modified the job and it is no longer available.

For more information on why jobs go so fast and useful tips on accepting them quickly, read more here: What Is The Fastest Way to Accept A Job?