you have to remember that with drush you are (most supposedly) in cli, so this should help :
http://stackoverflow.com/questions/1947395/how-can-i-debug-a-php-cli-script-with-xdebughttps://stackoverflow.com/questions/1947395/how-can-i-debug-a-php-cli-script-with-xdebug
also, try running the command with the option -d (--debug) - it should give you quite a lot of info about what is going wrong.
