Thick-film resistors (TFRs) consist of a percolating network of conducting oxide nanoparticlesdispersed in an insulating glassy matrix, whose resistive properties are dominated by quantumtunneling across insulating layers separating adjacent conducting grains. Tunneling processes are at the origin of the high sensitivity of the TFRs resistances to applied strains. We have measuredtransport and piezoresistive response for different RuO2-based TFRs as a function of metallic concentration x and RuO 2 grain sizes. The conductivity is shown to vanish as x approaches a criticalconcentration xc by following a power law with nonuniversal critical exponents, while thepiezoresistivity diverges at the same critical concentration. We argue that nonuniversality and diverging piezoresistivity have the same origin and arise from the highly fluctuation inter-graintunneling distances determined by the segregated microstructure of TFRs.