See this link under "Types of RAW files you can back up". Some RAW file formats, especially of new or very old camera models, aren't supported (yet) and they will be reported as "failed uploads".
A possible solution is to convert these to the "open" RAW format, DNG. This way you can keep your files in a RAW format, so you don't loose any quality, and they will be "safer" as on the long run DNG has a far bigger chance to keep being supported than a proprietary format made by one camera brand. You can convert your pictures to DNG using the free "Adobe DNG converter":
Windows version: http://www.adobe.com/support/downloads/product.jsp?product=106&platform=Windows
Mac version: http://www.adobe.com/support/downloads/product.jsp?product=106&platform=mac
Another work-around is to convert the RAW's to JPEG on the computer and upload the JPEG's. There are many free RAW converters. RawTherapee gives you more control. And of course many paid like ACR (Photoshop/Lightroom) and many others.
For RAW images, if a preview jpg is embedded, only the embedded jpg will be shown in Google Photos, not the actual RAW data in full resolution. See link
Two interesting articles about DNG vs RAW:
https://photographylife.com/dng-vs-raw
https://photographylife.com/why-i-no-longer-convert-raw-files-to-dng