Hi.
How check execute add_sub_row?
This function will return the new count of rows or false upon failure.
if ( false != add_sub_row('repeater_filed', array('field1'=>'value1','field2'=>'value2','etc'=>'etc value'))) :
add_sub_row('field1'=>'value1','field2'=>'value2','etc'=>'etc value');
$return_message .= 'You are add new row';
else:
$error_message .= "Error";
endif;
This code add 2 row.