Debugging the User Exit

Friends,

I have a user exit active , the main program is also active & generated.

In debugging, I come upto the place where the code is like

call customer-function '101' ...

but when I do F5 from here after that it doesn't go inside the code of the exit.

I have not come across this situation earlier.

What could be the reason in this case.

Regards

Prashant