Lenders can turn down applications for many reasons, including:
Having too low an income to prove affordability.
Receiving income in ways not accepted by that lender, such as benefits or commissions.
High outgoings or too many other debts.
Low mortgage to earnings ratio.
Bad credit score or history.
If you have been turned down on the grounds of affordability that by no means indicates that another lender won't accept your application.
Most of these criteria depend on the provider's internal lending policies, so it's often a case of finding a lender who we know you will be eligible with.