Are you right-clicking in the tree to create the new Article? There's a bug that prevents the Articles from showing up if you do that.
Some field for the Article in the modx_site_content table is not set correctly. I can't remember which one, but if you go into that table in PhpMyAdmin and sort by class_key, you should see the Articles at the top. You can compare the ones that show up with the ones that don't and fix it.
This is a bug. Check the URL parameters in the URL of the page it takes you to. I assume class_key is undefined. Should be fixed in the latest version of Articles if not I can share the patch
jpdevries