Is there away to debug a subflow script? I tried a reactive debug on the subflow script when it was called from main, but nothing happed.
No there isn't. You would need to either step through it within the editor; or, add logic that allows you to change the behavior (i.e. add an Accept step).
You could do this by an If step and changing the variable to True manually during a reactive debug. The If step would have an Accept step within it so you could call a trigger on a separate test application.
Another way would be to add the Get Trigger Info step and change the logic based on the trigger type (e.g. Cisco Reactive Debug Trigger). The same If step, or a Match step, could be used then.
Ultimately, the "new" way of doing this is not to use subflows but Trigger Application steps instead. Advantages:
-The application does not count toward your step count (it has it's own).
-You can call it synchronously or asynchronously. Note that for asynchronous it will be terminated when the triggering application ends.
-You can run a reactive debug on it like a normal script.
-Your code becomes much more modular and reusable.
-You pass data through the contact session instead of mapping it for each subflow call. This allows the information to be retained for the duration of the call.
The new approach requires CCX 5.0 and some practice to fully understand it (and sessions). Once you understand the details, it's a much better approach.