MissingMethodException when addin class tries to access, but method DOES exist in the latest referenced dll

Question

Lets say I have class as part of my executing code:

public class DataAccess
{
    //class definition...
}

and I have an add in class:

public class Tender
{
    public DataAccess DataAccess { get; set; }
    //other members and class definition
}

Well my problem is that I have a consuming (also add in) class like so:

public class TenderProcessor
{
    public void Process(Tender tender)
    {
        //having line below uncommented throws the following exception when 
        //this(Process) method is called: 
        //Method not found: 'namespace.DataAccess namespace.Tender.get_DataAccess()'

        this.DoSomething(tender.DataAccess, otherParametersEtc);    
    }
}

If this.DoSomething(tender.DataAccess, otherParametersEtc) is not commented, i get the Method not found Exception, however if I comment the line out, I can actually step into the Process method, hover over the tender object and see the value of DataAccess perfectly fine.

Can someone explain what's going on?