The Shift Frequency is set in the simulation properties dialog box, in the Solver tab, and only modes with resonant frequencies above this value will be computed and returned by the solver. Therefore, make certain that you set this value to be smaller than the expected lowest resonant frequency of the portion of mode spectrum you wish to analyze. Generally the eigensolver performs better when the resonant frequency of the mode being computed is close to (but above) the shift frequency. In simulations with multiple modes requested, the first several modes in a geometry may be far apart in frequency space. As a result, it may sometimes be difficult for the solver to accurately extract the high-frequency modes in the same simulation as the low-frequency modes. This will be apparent as convergence will require more AMR steps, and the mode frequencies may repeatedly change significantly between AMR steps. In this situation, the best results will be obtained if you run several simulations instead, where each new simulation contains a subset of the requested modes and the shift-frequency is raised appropriately.