RF Gain turned up all the way? Squelch not turned up too far? Antenna not being shorted to ground via an improperly assembled mount?
That the antenna may not have been tuned for minimum SWR yet will not affect its ability to receive CB signals.
If the RF Gain & Squelch controls are set properly, please verify them, try sticking just a short length of wire with the insulation removed from the end into the CB's center pin connector on its antenna jack and testing to see if you can receive anything. If it starts receiving signals with a piece of wire in the center antenna connector jack but not with the antenna connected, the antenna is shorted to ground via its antenna mount... likely from having been misassembled.