"I wish you a happy birthday!" should also be accepted!!!! - To say just "happy birthday" I have the impression that "生日快乐" would be enough! - It is very frustrating that most correct answers still not are accepted!!!
"I wish you" or "Wishing you" or even "Wish you" should be acceptable at the beginning of this sentence. Otherwise 生日快乐 and 祝你生日快乐 have the exact same translation... the additional characters should be addressed.