Google backup and sync all files in a folder have check marks but some files are missing in drive

Question

I created a folder on my computer beneath my google drive directory, and I placed 500 pictures and videos into it. Google backup and sync did it's thing, but when it was finished, not all the files appeared in my GDrive (i.e. in the cloud/on the web). I know this because I "downloaded" the folder which created a zip file that only contained about 430 files. This seems to be the only way to verify how many files are in a folder in GDrive. All the files on my computer have green check marks on them. I logged off and logged back on, thinking backup and sync would figure out the discrepancy, but it didn't.

What is going on? How do I get the files to sync? And more importantly, how do I know this isn't happening to other folders with my precious data that is supposed to be backed up in them?

I've googled the heck out of this problem (seems like a conflict of interest, TBH), and can't find any answers; almost like it doesn't ever happen to people.