If a job you saw earlier is no longer available, it was probably reserved by another Agent. However, this doesn't necessarily mean it's gone forever. If the other Agent cancels the job, lets it expire, or doesn't do it correctly so that it's denied, the job will automatically be made available again to other Agents in the area.
If many jobs of the same type all disappear at once, the quota for that job was probably met. A client will generally request a certain quota of responses, and once that many jobs are reserved or completed, the remaining locations for the job will no longer be available. However, if an Agent cancels their reservation or their submission is denied, the jobs will become available again, so keep checking back!
If you try to accept a job and you get a message saying it is no longer available, that means it has already been reserved by another Agent. It was showing up in your search results because the page was not up to date. Whenever you go to "Find Jobs," it's a good idea to hit the refresh button to make sure your list is current.