I can't save key press responses

Hello,

I'm using Matlab with Psychtoolbox and having problem with one thing. I have multiple tasks and I'm using keyIsDown for everyone of them. It works in other tasks but strangely it doesn't work in one of them.

In my task I want subjects to press '0' whenever letter X appears on the screen and press nothing if other letters appear. If X appears and they press '0' it's "correct", if other letters appear and they press '0' it's "incorrect". Also, if X appears but they press nothing I want it to be "miss" and if other letters appear and they press nothing I want it to be "correct no response".

When I use this code and save responses (I check responsedata to see what subjects did):

 t1=GetSecs;
    time=0;
    while time < responsetime % this is the maximum wait time
        [keyIsDown,t2,keyCode]=KbCheck;
        time=t2-t1;
        if (keyIsDown)
            if mytext1{i}=='X'
                responsedata1{i}.type='correct'; % correct response
                responsedata1{i}.RT =t2;
            elseif mytext1{i}~='X'
                responsedata1{i}.type='incorrect';% correct inresponse
                responsedata1{i}.RT =t2;
            end
            
        else %no button press
            if mytext1{i}=='X' %they are supposed to respond but didn't
                responsedata1{i}.type='miss';% no response
                responsedata1{i}.RT =0;
            elseif mytext1{i}~='X'
                responsedata1{i}.type='correct_noresp';% correct no response
                responsedata1{i}.RT =0;
            end
        end          
    end

for each target it either saves as miss or correct no response. Eventhough I press '0' whenever I see letter X, it still saves as "miss". I troubleshot other keys in my keyboard and the problem is with 'keyIsDown'. It doesn't recognize if there is a key press.

However, if I do it like this:

t1=GetSecs;
    time=0;
    while time < responsetime % this is the maximum wait time
        [keyIsDown,t2,keyCode]=KbCheck;
        if (keyIsDown)       
        time=t2-t1;

Then letters don't appear in a given time, only appear if there is a key press (here 'keyIsDown' recognizes if there is). I don't want that to happen. I want them to appear in a given time.

I don't know how to fix this. If there is someone who can show me what I'm doing wrong, I'd appreciate it.

Thank you.