Even a severely mistuned antenna will not cause that type of receive problem, mistuned antennas cause problems when transmitting. I'd say the problem is not likely to be in the CB radio itself, unless the user controls are messed up. If there is an RF Gain control, make sure it is turned up all the way. Adjust the Squelch knob just to the point the background hiss is shut off. Adjusting those two controls like that will ensure your CB is set to be as sensitive to a signal as possible.
Assuming you have the controls set properly & the problem still exists, I'd suspect the antenna system wasn't installed properly. The usual problem is a misassembled mount or a misinstalled mount. But if you're not comfortable with using a meter to check the antenna & its mount for shorts to ground, open connections, etc., I'd take it to a CB shop to have them troubleshoot it.
One last suggestion... take a pair of good sharp well-lit (use a flash if need be) close-ups of the antenna mount from both the top and bottom, then post them here. Quite often the problem can be seen from a good photo & diagnosed. The photo should be clear & sharp enough to be able to pick out individual antenna mounting washers & how/where they are positioned.