A common cause of impotence when you can normally get an erection and have sex is psychogenic erectile dysfunction.
Psychogenic erectile dysfunction refers to erectile dysfunction that is mainly caused by mental and psychological factors.
When the patient is having sex, excessive tension and excitement lead to over-excitement of the nervous system, which causes the arteries supplying blood to the penis to contract excessively, resulting in abnormal blood supply to the cavernous body of the penis, such as the penis can not be fully engorged with blood, which leads to insufficient persistence or hardness of the erection during sex, i.e. impotence. However, it does not affect the usual erection.
Psychogenic erectile dysfunction caused by usually can be erect, sexual life on impotence, go to the hospital to check the reproductive system usually do not detect organic lesions. Patients usually do not have positive signs, and will only realize that they are not able to do anything during sex.
If you can normally have an erection and then become impotent during sex, there may be other reasons, so it is recommended that you actively seek medical attention under the guidance of a professional physician.