If you "just tell her she needs to work it out and her Dad and I are always here for her," then maybe that's why she pulls away when she is having marital issues.
When I am having issues of any kind, I usually only tend to talk to people who are helpful. If someone said to me, "You just need to work it out," I wouldn't go to that person for advice any more.
While I think that mothers should not get enmeshed in their children's marriages, I do think they should be available to give advice when asked. My daughter does ask me for advice, and I give it when asked. I personally would have liked to have a wise parent to give me advice, not slowly figure it out all on my own, as I had to.
The other possibility is that this is just the way your daughter deals with things. As long as you are "very close" otherwise, I guess you just have to wait it out until your daughter solves her marital issues. Maybe she's talking to her girlfriends.
I think Michelle and the others make good points.
Suz - "glommy up in her business" lol.