For some reasons, threads that are either long or old (not sure which yet) dont show up in the search results.
IE, search for striker, and my buils diary doesnt come up, or 'warner' and cloudys R4 build doesnt show up, despite have comments recently added to them. Any ideas why?

Works for me
If I search Warner I get 181 matches. The warner r1 build diary is on the lat but one page of 20

I think your expecting a very specific result to come from a very general search

try either making it a more specific search (i.e. 'striker build diary' author 'loggyboy' )

or if you could try this for a more powerful search: linky

Just noticed your point about "despite recent comments"
Just because there were recent comments. the search stiill returns by date of the original thread, not the most recent post, hence cloudys build dairy being last but one page