Hi,
The behaviour that you reported is expected. Since you are calling the subvi in a loop, the execution control passes from the main vi to the subvi, comes back to the main vi, waits for the specified number of milliseconds, passes to the subvi again and so on until you hit the stop button. So in effect you do get access to the front panel of the main vi but its only for a split second until the control passes to the subvi again.
Try running your main vi in highlight execution mode with a constant 1000 wired to wait(ms) function. You will notice that that the subvi is being called indefinitely until you hit the stop button.This time, though, since you are waiting for a longer time(1 second) you have time to hit the stop button before the control passes to
the subvi again.
Regards,
Ankita A.
Applications Engineer
National Instruments