Problems with triggers and table lock

Dear guys,

we are using Postgresql 9.6.1 with Rubyrep in order to replicate some tables across two different, and remote, database
servers.
Since few weeks sometimes when inserting/updating some rows, the statement remains waiting and table seems locked for
insert/updates.

When i issue "select * from pg_stat_activity" no query is locked.
I only obtain this error messages when i kill (i also try directly from bash with pgsql) with "ctrl+c" the locked
insert:

ERROR:  canceling statement due to user request
CONTEXT:  while updating tuple (0,7) in relation "TABLE"
SQL statement "UPDATE TABLE SET FIELD=NOW() WHERE FIELD IS NULL"
PL/pgSQL function TRIGGER_NAME() line 3 at SQL statement

How can i see what is locking my table/tables?

Thank you!
F