If you sort your directory by most recent submission time and you get a lot of submissions, older submissions fall out of view quickly. The bumps system, which you can activate at Admin -> Settings -> Switches -> "Allow bumps", offers a way for submitters to get their submissions back to the top.

When bumps are activated, the user's member home area left side menu will have a "Bumps" option at the bottom of it. They click this to get to the bumps management interface, which displays all of their owned topics sorted by newest first. They can check a bunch of boxes and hit the bump button to manually bump their topics. In the right column they can choose to set autobumps -- when autobumping is set on a topic, the topic will be bumped back to the top every 24 hours for 7 days. After 7 bumps, a topic can no longer be bumped any further.

Usergroup permissions control how many bumps per day someone can do. Go to Admin -> Members -> Manage Usergroups -> (select group) to change the default value of 10 bumps per day to the value of your choice. There's also a usergroup permission to control who can do autobumps. Note that for usergroups who have permission to do autobumps a prompt will appear on the submit topic page asking if they'd like the topic to autobump.