In Calibre 5.19 (64 bit) on Windows 10 Pro (64 bit), Get Books doesn't necessarily find books where I know they exist. For instance, I just tried looking for a title of "project hail mary" (no quotes) by author "weir" (no quotes). Earlier today, the only match that showed up was for Google. I tried it again just now and at least Amazon is getting a match. But, nowhere else. Here are the existing product pages on:
Amazon
ebooks
Kobo
All three of those sites are configured and checked in the Get Books Stores area. Is this a known issue?