Use clock_timestamp().
now() is a traditional PostgreSQL equivalent to transaction_timestamp(), which is equivalent to CURRENT_TIMESTAMP. These functions return the start time of the current transaction. Their values do not change during the transaction.
statement_timestamp() returns the time of receipt of the latest command message from the client.
clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command.
For more information see the documentation.