Tests are completed on a "first come, first served" basis. When an email notification comes into your inbox, it's likely being seen by many other testers at the same time. By the time you logged in, it may have been taken by another user.
If the email indicated the test was specifically reserved for you by one of our customers, and you don't see it, contact the Support Team.
Remember, you have the best chance of seeing new tests if you stay logged in with your dashboard open. To qualify for more tests, always fill out the surveys in the blue bar at the top of your dashboard whenever they appear!