I just realized why River Song Doesn't know Clara Theory

In the Name of the Doctor when Clara and River meet at the Conference, River doesn't seem to know about her or met before - yet River knew Donna by name even though they never met.

When 12 and River meet in Husbands of River Song, he has forgotten who Clara is and looks like and only remembers the adventures after the events of Hell Bent, which means she was probably left out of stories, where he mentions Donna to River, over the 24 years at the singing towers.

What are peoples thoughts?