Sometimes you will see jobs disappear quickly — you get an email notification about jobs available, but then the jobs are not shown on the job board. This happens because most jobs are available to dozens or even hundreds of other writers, so they can get claimed very fast.
The email notifications can be very helpful; they give you a reminder to check the job board, and they let you know how busy your job board is. However, jobs get posted in real-time whenever clients post them. So, check the job board as frequently as you wish throughout the day for the best chance at claiming jobs before other writers do.
In rare cases, you might see a batch of jobs from one client disappear quickly. Another explanation for this is that the client has decided to remove a batch of jobs from the My Clients Job Board and direct assign them to one writer. Other times it might mean that the client simply canceled the project.