You are pretty much acting like a douche. Moving on.
So here's the why:
Each forum has it's own default display settings, included in these settings is a time frame to cutoff (From display only) old threads. Yours is likely set to "forum default" and therefor some areas of the forum have a very limited amount of threads displayed. For example, if a forum is set to show only the last days worth of threads (This includes threads with a reply from the current day) it will only show you the last day's worth of activity. This means that in a smaller subforum that already has pretty low traffic, you may only display a handful of threads from the last day, or whatever duration the subforum is set to. It is also entirely possible that you have your settings set at 1 month, 2 months etc, and your thread could simply be past that point and therefor not showing.
It has been mentioned already in this thread that you can change the display settings of a specific forum or subforum by scrolling down to the bottom of the page and changing the setting. This is a temporary fix and will reset back to default when you reload the page or navigate away and then return.
To permanently change ALL forums go to User CP (Link just under the Private messages button under your name) and go to Edit Options. From there scroll down until you find the box labeled "Default thread age cut off"
and change it to "Show all threads". This will override the defaults and permanently change the problem you were having. The other method is temporary and should be used if you want to keep defaults (Because it does keep things cleaner) but just need to view old posts for a brief time. Here is the setting you are looking for: