jl 500/1 powers up but no sound HELP

Hello i recently just got two jl audio 500/1. before i bought them i hooked them up in a friends car to make sure they worked and powered subs. Then the next day i put them in my car and they powered up but they had no sound. After they would produce sound i got a different amp and it powered the subs with no problem. So im pretty sure it doesnt have to do with my car at all. So anyone know what the problem might be????? Any advice would be great thanks