Version 5.0+ Only:

I used the edit folder "sort" page and now when I add new objects to the folder, the sorting isn't working like it is supposed to.

The control panel > folders > folder presentation switch is the master control for presenting objects in folders.

The folder-level "sort" page is an override for this master control
in case you need some folder to sort differently from the default.
What the folder-level "sort" page does is automatically apply sort
sequence numbers to override the control panel sorting, and any
individual user preference settings, and maintain the order you
specified.

This is less flexible than the control panel master sorting, because it will not dynamically update as new objects are added to the folder. For example, if you use Alphabetical sorting within a folder, and "newest first" in the control panel, and you add a new item to the folder beginning with "M," it won't appear in the M's, but at the
beginning or end of the list, depending on what type of object it is. When you add another new object of the same type, it will appear above the first new one you added, because it's newer.

In other words, old objects which have had the folder level sort applied to them will remain in that order until their sort sequence numbers are changed, and new objects added after that will obey the rules for the control panel master switch.
You can go back to the folder at any time and update the folder-level sorting to re-sort the objects.

To clear the sort sequence numbers and go back to complete control panel control, use the "default order from Control Panel" option.
PLEASE NOTE: If you even visit the folder-level sort page, sort sequence numbers will be applied to the contents and the control panel sorting will be overridden.