Just doublechecking. I've seen odd uses of things before and I hate making assumptions, so I tend to ask a lot of clarifying questions. Saves time in the end.

In your queue of 10,000+ items, are these previously picked up conversations that aren't closed yet, or is this a test queue that's pre-loaded and has never been worked against? Activation thread procedures will only pick up un-communicated/responding messages. If these conversations don't receive a conversation end message at the end, they hang forever. Also the queue doesn't immediately clear out closed conversations either, so this may depend on volume vs. the cleanup structure.

The first time your activation procedure comes online into a queue of that size with new messages, it should pick up its 1,000 messages. Otherwise it should only get (up to 1000) your new entries. This would be my most likely guess as to why you're not getting what you're expecting without further information.

Is there anything particular about this queue that might be considered non-default? I may be making some assumptions that are poor here as well if it's set up in an odd way.