1. User ended the item themselves.
2. Out of Stock Option was not checked in the eBay Site Preferences
3. The item was originally listed as a 30 day listing and user attempted to change it to GTC after the fact. This is technically allowed on the front end of ebay, but ebay only allows the item to be treated how it was originally listed.
So, in order to get these items back up, you must list the items again and make sure that GTC is selected before the items post. Also make sure that the Use Out of Stock option is checked in your ebay account. Then, you would need to edit the entries in Sku Grid and replace the old ebay item number with the new one so that it is tracking the correct thing. They should be okay going forward.