M-Files Web Preview shows some .pptx but not others

All affected documents can be previewed in desktop client without issue.  Size is not the issue--a 3,387 KB file will display in preview but a 195 KB file will not.  Password/encryption is not the issue--opening the file has no prompt for passwords. The preview pane in vnext says, "Let's try something else  You can open it to view it's content".  The preview pane in classic says "500".