zero values in statistic views

Hi all

I would like to see what is going on in a database using the statistic
views.

I am superuser,
stats_start_collector = on
stats_row_level = on
stats_block_level = on

select * from pg_stat_user_tables;
select * from pg_stat_user_indexes;
select * from pg_statio_user_tables
select * from pg_statio_user_indexes;

are showing zeroes in all columns where UPDATEs, INSERTs, block-reads,
  hits ... should be shown.

select * from pg_stat_database where datname = 'mydb';
shows between 20 and 60 numbackends and from 11 to 50 commits.

What could be the reason for this behaviour? What am I missing?

Thanks in advance
Conni