Associated Lead View missing companyname field when adding Columns

Hi,

I have Dynamics CRM in German so i hope I am getting the terms right.

I am trying to customize an associated view - LEAD in Dynamics CRM 2013. When adding a column the following Field is missing in the List of available Fields: "companyname". Why is this so? In other views associated with a LEAD I am able to add that column just fine?

Thank you in advance

boeschti