Sometime if you "pull" a slot over the hour on purpose (i.e. creating a route at 11:00 and then amending it to 10:55) all will be well but the slot will "go back" to 11:00 if you relinquish it - depending on how many others are free as you say. You may then not be able to pull it back into the 10's again. Could this be what's happened? You could have got the original slot ages ago from a flight you have changed/reassigned several times since.
Interestingly, and possibly related, often find that when I cancel a flight in order to use the slots elsewhere they do not show up at all! I remove the 'tick' to make sure the slots aren't permanently released and yet - no slots available (i.e. unused ones with a * next to them). If I recreate the exact same flight, it works - doesn't charge me anything for the slots but if I try to create a different flight, even at the exact same time, it can't find the slots. Weird.