Another thing I have noticed (testing some file uploads) is that if you lock the file upload topic in the archive and then do another upload:
a) the upload completes OK
b) a message is placed in the activity log to confirm which file was uploaded
c) no extra message is placed in the archive (as expected because the topic is locked)
d) here is no activity log entry to say that someone tried to send a message to a locked topic
I'm not sure if it is supposed to work that way or not in respect of (d) but it shows that locking the topic can also prevent new notifications being sent. I suspect the threading algorithm is playing a part here and a further upload in a few days time will generate another topic.