make sure that the commons: prefix is actually in your interwiki table
Yes, table wiki_interwiki has it, from $ grep commons mediawiki-1.11.0/maintenance/interwiki.sql ('commons','http://commons.wikimedia.org/wiki/$1%27,0),
Also, for easier handling of the interwiki table, try this extension
Can't I use any of the $ wc -l < mediawiki-1.11.0/maintenance/interwiki.sql 173 without dabbling in extensions?