Originally Posted by mherdeg

I dunno how this actually works, but I've certainly seen some "really full" flights not show up in availability searches -- like, they don't even show up in the search results that normally show "yes, there are four flights today and three of them are all zeroes". I've seen this happen for extremely full flights on united.com and also in the expertflyer.com "availability display" (even limited to nonstop options on a single carrier).