a couple of possiblities

If your form in its private data session makes any calls to global objects/methods and any of those issue set datasession command your global object may still have a reference to the private data session and it will not close properly.

If your form instantiates any object using createobj or addobject and those objects have references to them that are in existence when the form closes the form will not close properly.

Do you issue 'set datasession' command anywhere in your form? (=BadJuju)

Glenn