A user searches for a twin bed and gets nothing useful....

To diagnose why a search for ‘twin bed’ yields no useful results, you should systematically partition the potential failure points. First, check the posting count for each query term to determine if a misspelling is causing a retrieval failure. Second, investigate whether relevant documents exist under different surface forms, which would indicate a synonymy problem. Finally, examine the sense distribution of any documents that were returned to see if polysemy is causing the system to retrieve irrelevant content.