Runing statistics on a table partition does not finish

I have a 11.2.0.4 db.

In the database there is a partitioned table on a date column. This column is also indexed with an index partition. The partitions are per month.

After performance problems I found out that specific table partition and index partition based on May did not have any statistics. I noticed that last_analyzed=null on dba_ind_partitions and dba_tab_partitions. Previous month partitions were analyzed.

So I decided to run statistics on both.

exec dbms_stats.gather_index_stats( 'SCHEMA', INDEX, 'MAY2017' );

run in 1 minute

However I did not managed to run the statistics for the table partition