Service Reporting Issues

Not knowing how your online envrionment is configured I will provide guidelines on how I resolve this within our configuration.

If you are using Apps, I almost always see this data mismatch from a local file due to publishing an updated pbix to the workspace and not updating the app.  

If you are using a central pbi model saved in workspace A and building a report connected to this model saved in workspace B, the link between the Report and model is broken and the report published to workspace B needs re published.

If you hare using a scheduled refresh ensure your refreshes are succeeding

If you are using an on premise data gateway ensure that the data source is mapped and connnected to the data source

What we have built into our processess to assist in troubleshooting these items is to display a Data As Of on the page iteself.  The Data As Of represents when the datasource was updated, often by a max(date) in a fact table.  This self created Data As Of date then can be used along side the built in Data Updated avaialable in the service.  If the service Data Updated is > self created Data As Of then that tells us that the service successfully refreshed but our data source is stale (often excel sources).  If the service Data Updated is < the date we expected to be refreshed, we know something is wrong with the schedule refresh.  

Hope something here helps.