I have noticed this problem in the past, doesn't seem to be a problem with the script but a problem with the way Plus! itself hooks certain window messages.
If you disable LockOnClick in the XML (set Window/Attributes/LockOnClick to false) it seems to send the notifications correctly.
Obviously you can't move the window about by dragging it then so not much of a solution, sorry
Possibly a bug/limitation in Plus!?