This feature is available in Enterprise and Ultimate editions.
Requests disappear from the Request list after they are used to generate a story/backlog item. Where do they go?
When a Request is used to generate a story, one of the options when saving the story is Ok & Close Request. The Requests you want to see are most likely closed. Closed items are filtered out by default.
If you want to include closed requests in your views, see Viewing Closed Requests.