First of all, in this case, I suggest you go to the hospital for treatment in time, and it is best not to ask online.

Secondly, I can tell you some reasons and treatment methods about the natural inability of the arm to straighten through some personal experiences, but it is not necessarily accurate. I'm a sports student, and I have a sports rehabilitation certificate, but I don't have a doctor's qualification. I will answer your question from the perspective of sports rehabilitation:

First of all, check whether you have cubitus varus or cubitus valgus deformity.

This situation is generally caused by natural causes, that is, bone problems. If this is the case, it can only be corrected by surgery. Hospitals in small places may not have relevant equipment or professional doctors. You can choose to go to the top three hospitals in big cities with medium treatment, and you can choose a famous doctor for consultation. This situation is more professional, so I won't go into details.

Second, see if it is caused by excessive muscle tension.

This problem is mostly due to your muscles or tendons. If this kind of problem occurs, it can be properly alleviated through exercise rehabilitation, but it is difficult to completely cure it. Even if it is not completely cured, it can greatly improve the quality of life. If this is the reason, you can choose to go to the hospital to check the problems of muscle tension and muscle nerve, and carry out rehabilitation treatment under the condition that there is no problem with muscle nerve.

Third, the third problem may be muscle atrophy.

This kind of problem is also very common. If there is a problem of muscle atrophy, it usually involves your muscles and nerves, which will be more troublesome. Although surgery can be treated, it is difficult to completely improve your quality of life. The surgical plan can appropriately reduce your bending degree and improve the angle of motion of your arm in this way.

To sum up, the above three points may cause the arm to be unable to straighten naturally. Because I'm not a doctor, the answer I gave is just a reference. You can refer to this opinion and go to the hospital.