External psu for servos requiring thicker cables?

Hey i have 4x 20kg servos (each 4.8-6.8V) connected through 4 signal cables to the pi and then the + and - connected to an external power supply (5V 8A). Only 2 servos work, but its not the fault of the servos because i tried swapping them and they still work. Someone told me jumper cables maybe cant handle the 8A and so i should use thicker cables for that. Is that true?

Yes, when current flows through a cable, the cable heats up. So if too much amperage flows, the cable can burn out. Each cable cross-section can only handle a certain amount of amperes.