women gaining weight

I am 27 years old and I'm 5'7". I've had three kids. I was 116 lbs when I got pregnant with my first. I now weigh about 130 my goal weight has always been 145. I cannot gain weight by eating more or eating healthy/crappy foods. I've tried for years to gain weight, gym, protein shakes, healthy eating etc. I've also eat in abundance food that is supposed to help you gain. I've been to the doctor and I have nothing wrong with me. What else can be done to help me gain weight?