By any chance are you using it in an added site in a multisite install? If so, that could be it.
I’m not using multisite, just a single domain install.
For some reason the get_pool_ranking() function isn’t returning any results. Some things you may check:
1. Did you do a score calculation in the admin?
2. Are there any rows in the pool_wp_scorehistory table in the database? There should be some after a calculation.
3. If point 2 shows some rows: what is the ranking_id for those rows? Is it “1” (a.k.a. the default ranking)?
4. Check if the get_pool_ranking() function is called with the right parameters (league should be 0 and ranking should be 1, or at least the same as a ranking_id in your scorehistory table). Easy way to check: locate the print_pool_ranking() function in the class-football-pool-pool.php file and replace the first line of that function with this one:
$output = Football_Pool_Utils::debug( "league: {$league}, ranking: {$ranking_id}", "return" );
If you post your results it may help me in locating the problem.