A few questions.
1. How long has this refusal been going on?
2. Did she just have a baby?
3. Is she in good health?
4. Are there issues in the marriage she is angry about, which stay unresolved?
Answers.
1. If longer than six months see a marriage counselor.
2. It takes at least six weeks for her body to heal (after giving birth); and if she is breast feeding her breasts will be sore and tender for that entire time, also. And that is not even counting possible exhaustion from middle of the night feedings.
3. If she is not in good health there can be many issues affecting her desire in general and her physical capacity for sex. You can discuss those with any doctor.
4. Poor communication, which I'm guessing might be part of this since you have not asked her why she's uninterested, is something many women hate. If she keeps asking for the same things and doesn't get them, or if you break promises to her repeatedly, those will cause resentments. Some people express their feelings elsewhere. In this case it might be that she is expressing her resentments at you by sexually denying you. In that case the 'soreness' is possibly fictional. Other than emotionally being 'sore' at you.
Lastly - if she really is 'always sore' she should see a doctor, an ob-gyn specifically.