The short answer
Unresolved legal issues, lender concerns, missing documentation, planning restrictions, or unexpected survey findings.
Most delays trace back to something that could have been known earlier but wasn't checked for until the relevant stage of the process. Each one adds rounds of clarification that stretch out an already tight timeline.
Why this matters for your purchase
This is the kind of thing a PropM8 Property Intelligence Report is built to surface early, before solicitors, surveyors or lenders are involved, while it still costs nothing to walk away.