a Visa remarked by pencil with un-known reason what can be considered a valid Chinese Visa?

i am EU citizen (Germany) having Chinese Visa with twice time (2 times entering to China mainland). After the first time of entering to China was OK from Vietnam (HAN- PVG) but unfortunately my VISA was remarked by a pencil line of Immigration officer with unknown reason. With the second time i were rejected by the check-in agent in Vietnam on the same routing HANOI- SHANGHAI as the first time. Both two times i flied with Vietnam Airlines and were explained that due to my invalid and was cancelled by Chinese immigration officer. i am very confusing why my Visa having a pencil line could be considered it is a invalid Visa for using the next time. Could you please help to explain and let me know what are the requirements for a valid Chinese Visa and my Visa can be considered a invalid one or not? Many thanks for your help. Edwin