If you have defined the setup as an internal model it needs to be completely closed. There are 2 techniques I've found helpful.
1) change to an external model, then apply a very high flow or pressure to an inside surface of your model. When you try to solve Flow will show you where the open surfaces are due to high velocity escaping through the gap.
2) I often create fluid and solid volumes through check geometry. Then you can use the section view to scroll up through the model and look for regions where the inside and outside connect.
is a good first step
chris #1 is pretty interesting
ajay, if you are using flow 2014 and above you could use the leak tracker
also my suggestion would be to simplify your assembly down to just the main outside shell and see if that fixes the issue, then you can narrow down the component that is causing the issue. if you are new to flow, i would recommend doing some testing on known models like the tutorial models to get a hang of using the tools.