Can an amnesia patient lose some of their memory again? if yes, tell me how or why?

It's possible depending on what is wrong with them. People with brain tumours can of course get worse again later. Those who had brain removed, say to help epilepsy, can as well if the disease starts attacking another segment of the storage area. If the person has severe depression and is having a course of ECT (electric shock) to improve things they can lose a bit more memory following each treatment. Dementia is a progressive disease so people with that tend to carry on getting worse.