The 8 pcf value is the weight per unit volume for the balsa wood members. This is similar to mass per unit volume but expressed as a specific weight rather than a density. Make sure that in addition to this self weight of the balsa wood members, you apply a dead load of 1.44 psf to all floor areas. This dead load should be included in the seismic mass of the structure and therefore be activated as a lateral force when conducting a modal analysis or running a ground motion. If the 8 pcf self weight of the balsa wood is the only load considered in the analyses, your results will have very low displacements and forces. This could be the issue, but also keep in mind that a small-scale balsa wood model would have much lower displacements and forces than a real structure.