Functions not visible in pg_stat_user_functions view

Hi all,
I've notice not all my functions are tracked by pg_stat_user_functions view.
Interesting thing is similar functions in different db are tracked
correctly.

query:
SELECT p.* FROM pg_proc p
LEFT JOIN pg_stat_user_functions stat
ON (p.OID = stat.funcid)
INNER JOIN pg_language l
ON (l.oid = p.prolang)
WHERE stat.funcid IS NULL AND l.lanname = 'plpgsql'

gives non null output (50 rows in my case)
I am aware internal functions are not tracked, but in my case there are
user defined functions all written in plpgsql

any ideas?

params:
track_functions=all
PostgreSQL v. 9.2.2 on Windows 2008R2 (64bit)

Regards,
Bartek