In this paper we provide a detailed analysis of the energy position and type of transmission maxima in rectangular quantum wells (QWs), taking into consideration the difference of electron effective masses in the barrier and well layers. Particular attention is given to transmission maxima that are less than unity and the implications of effective mass variation. It is shown that, within theoretical considerations, there exist an infinite number of intervals of the strength of the QW in which such specific resonances occur, and that those intervals have finite asymptotic value for high QW strengths. (Contains 4 footnotes, 2 tables and 2 figures.)