The sonar pulse that a Garmin fish finder sends out can take a little time (milliseconds) to return. If you’re underway, in a few seconds you’ll be past the spot the fish finder just surveyed. If your screen is scrolling too slowly, you may not see that school of fish until you’re well past it. Increase the scroll speed of the screen. As soon as something interesting appears at the far right of the screen, slow down the boat.