What can I do if the car is still damaged after being repaired by the insurance company?

I got into an accident and had my car repaired at a recommended shop by the insurance. I got the car back and a month later it was having problems with the cooling system and the engine was overheating. The car was too hot to run so I checked the coolant and it was near empty. I re-filled the coolant and it was still overheating. I took it back to the shop that repaired my car because they were the ones that replaced the radiator after the accident. They took a look at it and said that everything was fine and couldn't tell me where the coolant was going. I asked them why is there not coolant there when they just repaired the car 3-4 weeks previously. They looked at it some more and still couldn't figure out where the coolant was going and ended up telling me I had a blown head gasket now. They’re telling me that the head gasket is blown because of “manufacturing defects” and is what caused the coolant to disappear into the motor. I believe that because the motor wasn’t receiving any coolant is why the head gasket blew. The shop and the insurance company told me that since there weren’t any leaks in the radiator and all the repairs seem to be done correctly then they couldn’t do anything for me. So I took it to another shop and they ran their test and it concluded that the radiator cap wasn’t holding pressure. The pressure test came out to be 12 psi when it’s supposed to be 14-18 psi. With radiator leaking pressure the coolant can’t get through is getting pushed back into the reservoir, which holds the coolant until it is used by the radiator. They concluded that the head gasket got too hot because it wasn’t receiving proper coolant and may be from the cap leaking pressure. I talked to the insurance company again and they had someone come out to look at the car. They ended up telling me that they couldn’t cover the head gasket repairs because they couldn’t relate the accident to the head gasket being damaged.