[wix-users] FilesInUse event

[Roland K]

I have a custom bootstrapper with a msi inside.

If I wanna update an existing installation, but there's still an
application open (or a file in use) which should be replaced, I wanna show
a dialog, so the user can decide if he wants to retry, ignore or cancel it.
Sometimes the "FilesInUse" event of BootstrapperApplication gets raised so
I can show the dialog, but sometimes NOT.

Why?

What can be the reasons the event is not raised?

Thanks in advance!