Can you check under UserCP
Setting and options -> Edit options
On this page
Thread Display Options -> Default Thread Age Cut Off
What is the setting on your page, The pull down has several on options.
Starts with Use Forum defaults, and then increments from last day up to show all threads.
If you have any other setting, try changing it to show all threads, and try the search again.
I have searches show up threads from 2003 quite often when looking up old Electrical items in a specific ( electrical ) forum.
Also you can search from the black menu bar, just to make sure with the engine rearrange it did get put in the correct place ( if past the sub forum change ).
It is the last item showing on this page ( from the other question, ignore the red box )
