It sounds like your subVI is still running. The output data will not flow from the subVI until it has finished executing. Make sure when you 'close' this subVI you actually press a button to STOP the subVI's execution loop - otherwise the subVI will still be running in memory, but without showing a front panel.
If you still have this problem try using execution highlighting. It may take a while to run through the code, but you can see exactly what is happening to the dataflow and whether your subVI has actually finished executing or not.
Kim