The heat capacity and neutron powder diffraction measurements of the double perovskites La2NaB′O6 (B = Ru5+, Os5+) with S=3/2 on the B¿ reveal an incommensurate magnetic ground state for B¿=Ru, but no clear sign of long range magnetic ordering for B¿=Os. This behaviour is attributed to the large monoclinic structural distortion that weakens the nearest neighbour (NN) superexchange interactions, presumably to an energy scale that is comparable to the NNN superexchange. This provides a novel mechanism for frustrated magnetism in the double perovskite family. We therefore propose to investigate La2NaB′O6 (B = Ru5+, Os5+) using neutron scattering and muon spin rotation measurements to understand the origin of frustration and complex magnetism in these compounds.