Statistics tables not being updated anymore

Hi.

v9.6.9

Statistics views like pg_stat_*_tables, pg_stat_*_indexes,
pg_statio_*_tables and pg_statio_*_indexes aren't being updated anymore.
Specifically, all counter fields are 0, and date fields are blank.

The first thing I checked was postgresql.conf (but it hasn't been modified
since December 2018), and track_activities is turned on. Also, I connect as
user "postgres", so it's not a privileges problem.

Where else should I look?

track_activities                    | on
track_activity_query_size           | 1024
track_commit_timestamp              | off
track_counts                        | on
track_functions                     | none
track_io_timing                     | off

Thanks

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