Everything below the top level work item needs a parent for the backlog structure to work correctly. So as you create new work items, everyone should keep an eye on that. ScrumMasters should facilitate their teams backlog work and monitor their Backlogs for data quality problems like these.
Some info on the VSTS Work Item Visualization extension, providing some quick tips and links to learn more. This VSTS extension allows you to visualize relationships between work items from within the work item form.
Documents the VSTS Product Backlog Hierarchy used for sophisticated products and portfolios of products, and how you can use the levels of the hierarchy and the work item types.
Quick tips on writing good Epics, Features, Stories and any other work item types and using the INVEST technique to break down work in to smaller pieces.