parallel processes blocking other parallel processes in 11.2.0.2 data warehousing database

Hi Experts,
We recently noticed that when we run large query in parallel mode, we see one sid blocking a few other sid running the same query. Here is our current parallel process parameters. Any suggestion what we set wrong?

SQL> show parameter parallel

NAME                                 TYPE        VALUE
------------------------------------ ----------- -------------
fast_start_parallel_rollback         string      LOW
parallel_adaptive_multi_user         boolean     TRUE
parallel_automatic_tuning            boolean     FALSE
parallel_degree_limit                string      CPU
parallel_degree_policy               string      MANUAL
parallel_execution_message_size      integer     16384
parallel_force_local                 boolean     FALSE
parallel_instance_group              string
parallel_io_cap_enabled              boolean     FALSE
parallel_max_servers                 integer     160