The heartbeat goes off once every 5 minutes for each stock. The exact time is random, so we spread out the stocks evenly over time. Your strategy doesn't work because you are looking at a one minute period, so only 1/5 of the stocks you're looking for will show up.
Try a different alert other than the heartbeat. Example: Running Up/Down alerts. This will help.
Another suggestion: What about selecting numerous alerts with a Gap filter and setting the view mode (right click and select this option) to Group and Sort by SYMBOL. Just a couple of thoughts.