Using Derived Class To Satisfy Interface Requirement?

I have an interface (called IPersistentBusinessObject) with a required function that looks like this:

Function GetFields(ByVal fromDatabase As clsODBCDatabase) As enumSuccessFailure

In a class, I try to satify the requirement with the following:

Public Function GetFields(ByVal fromDatabase As clsDatabase) As enumSuccessFailure Implements IPersistentBusinessObject.GetFields

In this example, clsDatabase inherits from clsODBCDatabase. So I'd expect this to work since clsDatabase does everything clsODBCDatabase does and more. However, it does not work. The compiler complains that I haven't properly satisifed the interface requirements for GetFields. Can anyone tell me why this does not work?In slightly different but related example, I have an interface that requires a function called 'AddItem' as in:

Sub AddItem(ByVal objObject As IPersistentBusinessObject)

In my 'building' class, I attempt to satisfy this requirement using:

Public Sub AddItem(ByVal building As clsBuilding) Implements IPersistentBusinessCollection.AddItem

In this example, clsBuilding implements the interface IPersistentBusinessObject. So I'd expect this to work since clsBuilding satisfies the interface contract. However, it does not work. The compiler complains that I haven't properly satisifed the interface requirements for AddItem. why this does not work?