OpenSesame requires repeated keybord responses

hi everyone,

i'm trying to built n-back task. because i have no idea about pyton coding, i tried to built it manually. in this purpose, i defined a 'letter' variable -which is shown via scatchpad- and a correct_response variable. i ordered the letters manually and assigned the correct responses according to a standard 2-back paradigm. (btw in the new_loop i changed the order from random to sequential). i also defined feedback via skatchpad same as on the tutorial video. when i run the experiment, everything looks like ok but when i try to response to a single letter with my defined keypress(n and m), it doesn't screen the next stimulus (next letter). it screens the next letter if only i press the defined keys twice. what possibly might be wrong?