why the field SQL_TEXT of performance_schema.events_statements_current often empty?

We used the percona server 5.7, and tried to get the sql text with the field trx_query by the following sql ,however I often found this field is empty, what's the reason and how to avoid get the empty sql text?

SELECT b.id AS processlist_id, trx_id, user, host, a.trx_state
    , unix_timestamp(now()) - unix_timestamp(trx_started) AS trx_time
    , db, trx_rows_modified 
  , SUBSTRING(CASE 
                  WHEN trx_query IS NULL  THEN c.SQL_TEXT
                    WHEN trx_query IN ('commit')  THEN c.SQL_TEXT
                  ELSE trx_query
                END, 1, 10240) AS trx_query                
FROM information_schema.innodb_trx a
    INNER JOIN information_schema.processlist b ON a.trx_mysql_thread_id = b.id
  INNER JOIN performance_schema.threads  t  ON t.PROCESSLIST_ID = a.trx_mysql_thread_id
    INNER JOIN  performance_schema.events_statements_current c on  t.THREAD_ID=c.THREAD_ID 
WHERE trx_started > '0000-00-00 00:00:00'
    AND unix_timestamp(now()) - unix_timestamp(trx_started) >= 10
    AND trx_tables_locked > 0
ORDER BY trx_time DESC
LIMIT 0, 30;