My female rat doesent gain weight

Question
I have 2 rats. One male, hes about 12 weeks old and hes very friendly, happy, and is good and "round". And i have a female, shes about four months old and she had a rough kit-hood. when she was very small she got loose in the house and we didn't catch her in about a month after we lost her, and she hasn't been the same since. even though i spend lots of time with both the rats, and every time i pick her up she goes still. Not "stiff" but just still. She wont really open up to me. But that's not my main problem. The male gains weight regularly but shes always kinda skinny. I keep getting the feeling that shes depressed or something. I really don't think the male is hording all the food. I also can't seem to get her to go into heat ether. She pretty much raised the male from a young 3 week old kit. I'm not sure what to think or do. Please help me!

Answer
I am hoping you dont have the two rats together, because if you do, she will be pregnant before you know it unless you tell me one of them is fixed?

Also, how often do you feed her? Constant supply of food? What diet do you use?

Again, unless one of them is fixed, its wise to get them in their own cage and I mean now, if she isnt already pregnant.