Why a variable randomly is not updated in MATLAB GUI using guidata function?

Hello, I have a problem in a GUI made using MATLAB Guide.

I have some timer functions which are executed sequentially. There is one variable (handles.UserData.C_cnt) which is shared between the timer functions. The variable is not updated after some random number of the execution of the timer function and I cannot figure out why this occurs. Could you check if the following structure I use for updating the variables inside a timer function is correct?

function timer_fcn(obj,event,hObject,eventdata)
handles = guidata(hObject);

handles.UserData.C_cnt = handles.UserData.C_cnt+1;
guidata(hObject, handles);

[Adam]

That single piece of code looks fine. How it interacts with the rest of your code we don't know though. I can fairly certainly say there is nothing 'random' about it though. Undiscovered, perhaps, but it will be purely logical unless your program actually includes randomness.

[RZM]

I agree. Some experts recommend not to use global variables and they are evil so I tried to avoid them in GUI. But here when I use global variables it works fine so in this case I call them angels. :D I have also used eval function which is not recommended, maybe this random problem came from that but I had no other choice.

[Stephen23]

@RZM: please upload your code by clicking the paperclip button.