If your application was declined, your lender may be unable to give you specific reasons why. Lenders use information from your credit report, alongside your income and expenditure data to make a lending decision. If you feel that there has been a mistake, please contact your lender - their contact details will appear on the email informing you that your application was declined.
You can also contact the Credit Reference Agencies if you think there is a mistake on your credit report:
Experian
Equifax
TransUnion