Sudden occurrence of a large forced oscillation (FO) is a challenging problem for power system operators, especially when the forced oscillation is resonating with a natural system mode. Among the methods for oscillation source location, the energy flow method has shown promise for detecting the source of a natural oscillation as well as a forced oscillation. In this paper, we analyze the performance of the energy flow method in depth by evaluating the expressions of incremental energy in one period of the forced oscillation and by looking at a wide range of forced oscillation frequencies. We show that the energy flow based method can misidentify sources of forced oscillations. That is, the method may not be able to locate the true forced oscillation source as well as identify a non-source device to be the source. Examples are shown for systems with different damping levels of system modes and for different load composition types.