I converted a board from phpBB2.

I then went on to try and create a search index. Which got stuck. Please see

http://www.phpbb.com/community/viewtopic.php?f=46&t=1571295

For more details

I worked through the code and found that when this is saving the state, it is not setting it as a dynamic value, and therefore is getting cached and being reloaded as the same value

A simple

UPDATE phpbb3_config SET is_dynamic = 1 WHERE config_name = 'search_indexing_state';

Fixed this.