What causes sync'd excel to miss Forms responses?

Hi All - 

I'm having trouble with seeing all responses in a sync'd excel sheet. I created a "New Group Form," tested it out, and confirmed that when people respond to the form, my sync'd excel will update in real time (~ish), which is perfect. I would like to use this form indefinitely to replace a process that is currently very manual. But now after a couple weeks of success, I'm missing two responses. I have earlier responses, I have later responses, so the form didn't completely unsync. 

I've read through the microsoft troubleshooting, but it doesn't have a ton of details...

• Excel workbook is corrupt - doesn't seem to be.
• Excel workbook is too large - nope, under the limit. 
• Excel workbook is locked or checked out - we're not requiring anyone who might have access to the excel to do this; is this something that could happen by accident that would temporarily prevent synching? 
• Excel workbook has been manually edited - well, in my case, to not edit the excel workbook at all would defeat the point, but I've edited sync'd excels from forms before with no issues. Anybody know more specifically what editing actions will cause issues? 
• Excel workbook is IRM protected - presumably this would prevent it from ever synching again, and wouldn't be a temporary problem. 

The only other thing I can think of is that maybe some of the responses contained characters that break the synch - is that even possible? 

Any help is welcome. 

Thanks!