What you a touching on is a historically a huge and difficult part of this job - the bridge between director and cast.
I have made a slight career about working with directors that casts sometimes don't "get", and I can offer some hints, but without knowing your own management style, you need to take this with a grain a salt.
1) Like Gil said, you need to know in what manner people are talking to you. You need to ask them if they are just blowing off steam, or if they are asking you to take action. Sometimes a cast just wants empathy.
2) I think it's a mistake to "blindly" up-sell the director, but you should never through him under the bus. If you are just blindly up-selling him, it sort of makes you look like you are a bit naive and blind to the real issues that is part of your job to deal with.
3) You need to have a frank and open discussion with the director about the issues, and like most things, try to separate the issues from the "Director" - like the cast is having some problems with feeling like their time is being wasted - not, "You, Mr. Director fellow, are wasting time." Even if they are small issues, think of way to bring the small issues up in small ways. Better to tell them about a couple of grumbles then blindside him with a huge problem. If no one is talking to him about it, how can he fix his behavior?




? I am sure the actors are going to him. And remember, he is having to make 100's of little decisions - he may not know if the attitude he is getting is about his behavior, and not about a directing choice that actress disagrees with.
4) THEN, I try to offer a solution based on what I can do. So, telling the director the cast is having some problems with feeling like there time is being wasted - is there anything I can do to help the director break down the rehearsal schedule into small blocks so that we are wasting so much time. (In the end a happier cast is better cast . . . most of the time.)
5) If the cast and the director both see you trying to work towards a solution to the problem, you will find a) you are above the fray, but b) you are bringing the issue to the director's attention, coming up with a solution and NOT taking personal sides. If you can make headway, then maybe getting a respected cast member to talk to the director may help.
6) If the director ends up being "Stubborn" and not willing to change, then, well, welcome to theatre. It's not fair, it's not a democracy, and the director has an extremely difficult job . . . his style and organization may not please many people or any people, but it his method of working and it works for him, so . . . unfortunately that is the way it is. I know that sucks, but this very issues I would say is why 30% of professional stage manager leave the business - not wanting to deal with the silliness of some directors.
Good luck.
(Completely aware I made this director a guy, I know not all directors are male)