Another thing to consider is the repo you are using - they may not of synced to that mirror at that time.
I just checked Scribus (126.96.36.199) and I can see it - so possibly change repo (I'm using nluug.nl)
The apps are first added to the PASS server. After a day or two they get synced to ibiblio, from which all other mirrors sync, each on it's own schedule. When it's announced that a new, or updated package has been added, it should be on most, if not all, mirrors within a week.
If you let update-notifier do the checking for you, you'll have a good idea when your chosen repo last synced, by being notified that new upgrades have arrived. If the application you are interested in is new to you, and not yet installed, right after doing the upgrades would be a good time to see if that particular package has arrived at your mirror.
If you check the package listing from update-notifier, the first window displays the upgrades you need, the second window shows new packages that have arrived, that you do not have installed.