Numerous things can delay closing. The most typical are..
Financial/loan issues: Lender issues with paperwork, underwriting, appraisal, etc.
Inspection issues: agreed repairs did not get made, unsatisfactory walkthrough
Title/Lien issues: title search was scarred, unknown liens were discovered
Due diligence: People involved in the process not doing their job efficiently. Keep in mind successful closings start long before the actual closing time.
Moving issues: Seller has issues with new living arrangements, proper contingencies weren't lifted on future living arrangements for seller.