How to you tell your sensei (with respect) their waza isn't good any more, that it has deteriorated and become stagnate. Because you respect your sensei your just not going to abandon her, after all the time and effort she has put into you. You want her to regain the lost skill, she once had.
It would be helpful to know what you mean by "lost skills", "deteriorated" and "stagnate". It would also help to know approximately how long you have been studying the art. It is hard to determine why you are experiencing what you are.