Threads are locked either

1/ by Admin
or
2/ when they reach 19 (?) replies plus the base note - which I think is a feature of the Board

I have seen "new threads" appear with a pointer back to the locked thread - I suspect this is when Admin spots that an active thread has locked :-) example http://www.forum.forest-of-dean.net/index.php?id=13966

I guess Admin can "wake this one up" with a linked Thread
http://www.forum.forest-of-dean.net/index.php?id=13920