There could be a number of reasons for this, but starting with filtering is a good first step.

The “Activities” lightning component for the page layout by default is filtered to “Within 2 months”. Users can adjust their preferred settings to show more or less. Page load time will be impacted slightly, but by selecting “All time” all, possible activities will be represented.

For any setting changes that would like to be maintained, “Apply & Save” should be selected. For temporary filtering “Apply” should be selected.