The main reason references fail are down the below:

1. Salary and job Security - All references will take into account how much you earn and weather your job is permanent or part time (zero hours contract). Most referencing companies work on a factor of the yearly rent x 30 is the minimum the combined earnings of all tenants need to be. In addition they need to ensure as far as possible that you have job security for the amount of time you are wanting to rent the property for. 

2. Bad Credit - This is not the same credit check as when you take out a loan, what the referencing companies search for are outstanding or current CCJ's (County Court Judgments made against you), Any bankruptcy orders and criminal charges / convictions. 

3. Landlord Reference - A previous landlord reference is always required to check that as a tenant, you looked after the property and paid your rent on time. If you have not rented before, the landlord may seek a guarantor to ensure any damages are covered and rents are paid on time. 

If all three criteria are met, the reference should pass. If you do fail the reference, this will be discussed with the landlord and possibly a guarantor sought.

You must disclose anything from the above that may affect your reference at the point of viewing the property.