With SubFinder or SmartFindExpress, you may receive an alert or see a job on their site, but if someone else starts viewing it first it will become 'locked.' During that time you will not be able to view or accept the job. You will only be able to view that job once it becomes 'unlocked' again on their site AND if you are the first one to click on the job at that time. The best option is to continue clicking on the link until you are able to view and accept it. 

For other useful tips on successfully accepting jobs, read further here: What Is The Fastest Way To Accept A Job?