Using Newsbin x64 on Windows 7 home premium:
Newsbin v. 6.0: 00 FB 77 E0 74 2E
I have a groups folder with 10 newsgroups. After I download the latest headers and look at the list, even after I re-sort it, some headers are not displayed. (I have the <1 day old filter active). If I toggle the filtering - alternating between some random filter (like multimedia) and then go back to "None" on the filters pulldown menu - taddah! Many many more postings show up. I still have the <1 day filter active.
This isn't a one time thing. It happens pretty much every time, I think.