A comparative study of electronic properties of the two isostructural narrow-band semiconductors FeSb2 and RuSb2 by means of experimental NQR spectroscopy as well as theoretical ab-initio band structure calculations is presented. The temperature dependence of 1/T1 consists of two distinct intervals: above 40 K (HT) with activated behavior for FeSb2 with D/kB @ 450 K and below 40 K (LT) with smooth maximum at 10 K. Here the relaxation is governed by in-gap states. We propose the model of inherent Sb-deficiency (as prepared non-stoichiometry) of both FeSb2 and RuSb2 as a possible reason of the in-gap states. This results in creation of a small portion of Fe (Ru) ions possessing formal oxidation number +3 with d5 configuration and forming narrow energy level of localized S = ½ spins near the bottom of the conduction band. Due to much higher gap value in RuSb2, the activation mechanism for RuSb2 in the HT range is inefficient and the 1/T1 dependence in the HT range is more close to T2 behavior characteristic for phonon relaxation mechanism by two-phonon (Raman) scattering.