SQL unpredictable failure - mysql

Question

I wrote an SQL stored procedure, that's called trough an SqlDataSource element (in a ASP.NET / C# web application).
To be more precise the procedure is called from within a trigger of tableA bound on the AFTER INSERT event:
CREATE PROCEDURE `create_checklist`(IN _id_A INT(10))
BEGIN
DECLARE _mode CHAR(4);
DECLARE _id_default INT(10);
SELECT
mode
FROM
tableA
WHERE
id = _id_A INTO _mode;
SELECT
id
FROM
tab_doc_type
WHERE
def_value = 1 INTO _id_default;
# LOCK TABLES TableA WRITE;
# START TRANSACTION;
INSERT INTO checklist
(id_type, id_A, priority)
SELECT id_doc_type, _id_A, priority
FROM tab_checklist
WHERE A_mode = _mode;
UPDATE
checklist
SET
not_needed = 1
WHERE
(id_A = _id_A ) AND (id_type = _id_default);
# COMMIT;
# UNLOCK TABLES;
END
CREATE TRIGGER `tableA_AINS` AFTER INSERT ON `tableA` FOR EACH ROW
BEGIN
CALL create_checklist(NEW.id);
IF NEW.mode = 'X' OR NEW.mode = 'Y' THEN
CALL create_booking(NEW.id);
END IF;
END
that should create the checklist entries for an element of the tableA and then update one of the checlist entries with a flag (not_needed ) with a predefined value.
My SQL backend is a MySQL server.
This works in most of the cases but users reports that 'sometime' the procedure fails without errors being shown, the checklist is not filled up so I'm assuming that INSERT command is not executed.
I'm not able to reproduce this behaviour.
I've been notified from a user the value of an _id_A on which the procedure failed and triggering it 'manually' from the MySQL SQL Editor worked withtout problems.
I tried to enclose lines of codes 'writing' data in table with LOCK TABLES .. UNLOCK TABLES and/or START TRANSACTION ... COMMIT without success. In both cases it seems that stored procedure cannot use such commands.
Anyone here is able to locate a possible failure reason?
There's a way to fix this?
In short there's a way to call the stored procedure from within the trigger and avoid race condition so the checklist table is correctly populated every time?