Why can't some players find any Steam sessions (when most can)?

Hi folks. My multiplayer game is about to be released on Steam. Most of our beta testers had no problem finding sessions, connecting, and playing. So this isn’t one of those “put-the-steam-ID-in-the-right-place” issues since it works for most people.

The trouble is, some Steam users just don’t find any sessions and no one can find theirs, rendering my game unplayable. They have other multiplayer games that connect fine, but I’m assuming THOSE games don’t use the Steam Subsystem.

I have increased the session results to like 200000 just to cancel that possibility out. So what else could be causing some Steam users not to find any sessions? I’d say it’s about 10%. It’s not geographical either. Some live in my city.

Thanks in advance!

Any fix to this?
What did you do?