values from now() in the same transaction

I created a function with VOLATILE directive. it's
body looks like shown bellow

------------------------ cut start
begin
insert into monitor(ts, c1) values(LOCALTIMESTAMP,
'Step 1000'); -- start time

-- query below runs for 20min
insert ito t1 select * from big_table

-- this timestamp should be bigger by 20min than start
time
insert into monitor(ts, c1) values(LOCALTIMESTAMP,
'Step 1001'); end
----------------------- cut end

The value returned by LOCALTIMESTAMP function is the
same in both places despite that actual interval of 20
min between these calls.
I tried function now(),current_timestamp() but all of
them behave similar.

I don't believe that it's bug, probably it's a feature
of the postgreSql database.

Is any way to insert a timestamp within the same
transaction that would have current system time (not
time of the beginning of the transaction)?

With other words, I would like to see different
timestamps on first and last timestamp.

Thank you,
Vladimir