If you received an alert, tried to accept, and still lost the job, there are a number of reasons this could have happened. Most likely it was due to the way your substitute-service operates. 

Aesop: With Aesop, our servers are only able to check for an available job every sixty seconds. In between that time, when the server is in between searches and a job comes up, someone else will be able to access it. So when the next sixty seconds come, the job will still show as available while that person in accepting it and when you try to accept it, it isn't available anymore. 

SubFinder: With SubFinder, you may receive an alert for a job, but if someone else starts viewing it first it will become 'locked.' During that time you will not be able to accept the job. You will only be able to view that job once it becomes 'unlocked' again on the SubFinder site and if you are the first one to click on the job at that time.

So it's helpful to understand that it's the nature of the sub systems that even in accepting jobs, you'll face some competition. However, our alerts still are giving you the opportunity to be alerted to more jobs more quickly than if you were relying on your computer alone.