We present results of an investigation of the long-term variations of the amplitudes of the three strongest modes of 16 (EN) Lac and of the period of the strongest mode of ν Eri. We show that in the case of the first star there are feasible explanations of the observed variations. The explanations involve non-linear interaction between pulsation modes (the first two observed modes) and beating between two non-interacting modes of constant amplitudes (the third observed mode). In the case of the second star, however, we can only indicate the difficulty of accounting for the long-term period variation.