User Metric Report

Hi RoyC_ ,

This might be a bit about which version of the Usage Reports you are using.  The newer version has some better logging built within it.  That said, if a user is viewing and not interacting with a report (particularly if they just have it up on their browser and leave it open for long time periods) and don't actually edit or otherwise interact with it, then the Power BI Usage Metrics won't report it as an interaction.  

Power BI Usage Metrics tracks opening, navigating, interacting, export, filtering or slicing but not necessarily just viewing.

Also, the user has to be using the same sign in that you are expecting to see.

Most accurate way to measure events is actviity events - might be an overkill for your scenario - but pretty accurate - https://learn.microsoft.com/en-us/rest/api/power-bi/admin/get-activity-events