The breakdown of the principle of reciprocity is a well-known phenomenon of nonlinear systems. A structure or system is said to exhibit reciprocity when the response at some point j to an input at some point i is identical to the response at point i when the same input is applied at point j. This paper seeks to explain this phenomenon by adopting a functional series representation which describes the input-output relationship. The frequency-domain Volterra Series representation utilises Higher-Order Frequency Response Functions (HFRFs) (generalisations of the linear FRF) to explain the behaviour of nonlinear systems. This breakdown in reciprocity may be observed through a breakdown in symmetry of HFRFs.