First, check your build settings for the subVI. Front panels of subVIs are normally deleted when you build an app. I'm assuming that you're using the GetPanelImage method. If there's no front panel, the method will fail. Click the VI Settings tab, and look at the settings for the subVI. You can choose to keep the front panel by clicking the Edit Build Settings. Second, how are you referencing the subVI? Paths to a subVI are different when you do a build. For example, if in development mode you refer to c:\folder\subVI.vi, when built, the path becomes c:\folder\main.exe\subVI.vi.
