I don't doubt that your application is working but I do doubt it is working exactly the way you described.

When you main form is up and running (as a top-level form) and one of your subsidiary forms is up and running (also as a top-level form), can you switch back and forth between them. If so, they are not modal.

When your forms are active, run the Debugger and check the Windowtype property. Is it 0 or 1? If 0, it is non-modal.