A software product-line is a set of software intensive systems sharing a common set of managed requirements. A technique for requirements analysis of single systems, but rarely applied to product lines, is Problem Frames. Problem Frame theory omits support for the management and exploitation of product line requirements variability. This paper presents some notation to support requirements and machine variability in problem frames. It also describes some transitioning rules between requirements and machines that can help to manage product line evolution. We illustrate our ideas using a mobile phone worked example.