Consider this scenario:
You have a SharePoint 2013 environment with the PowerPivot 2012 add-in for SharePoint installed. You have non-PowerPivot workbooks in your PowerPivot gallery, and you have successfully set up scheduled data refreshes for those non-PowerPivot workbooks. When you upload the non-PowerPivot workbooks to the PowerPivot gallery, thumbnails for those workbooks are generated successfully. Then when you run a scheduled data refresh for a non-PowerPivot workbook, the thumbnail for that workbook may give you a red x.
An example of a non-PowerPivot workbook is a workbook that was created with a regular non-PowerPivot connection in it to a SQL or OLAP data source (i.e. the PowerPivot add-in was not involved in the creation of the workbook, and the workbook has no embedded PowerPivot data in it).
For all versions of PowerPivot for SharePoint, scheduled data refresh is not supported for non-PowerPivot workbooks.
In the scenario described above, scheduled data refresh for non-PowerPivot workbooks may work, but this functionality was not tested or intended to work.
This unsupported refresh functionality may have unintended side-effects, such as the thumbnails for the workbooks being broken in the PowerPivot gallery when the scheduled refresh occurs (and getting a red x instead of properly-generated thumbnail images). PowerPivot scheduled data refresh was only intended to be used with (and is only supported for) PowerPivot workbooks (i.e. workbooks that were created using the PowerPivot add-in in Excel). It is not generally recommended to use unsupported or unintended functionality in a production environment.
One possible workaround for this unsupported scenario is to open the workbook in the Excel client, refresh the workbook, and then save the workbook back to the PowerPivot gallery. Another workaround is to make a small change to the title (not the name) of the workbook. These workarounds may or may not work, and they may or may not be appropriate for your particular needs, and are unsupported, but they could be useful in some situations, such as for when non-PowerPivot workbooks are set to refresh less frequently (ex. one time per month).