they can only appear in her dreams...
and as it was shown a bit more, in different forms, like their age...
they can only show themselves through memories really, and yet they have to have a valid reason to do so.
I'll explain a bit more at the end of part 4

It'll make a bit more sense once you kinda read a part in the last bit of the (end of the) chapter
And now that i read over a dream in a future chapter (yes, dreams throughout the story, no secret to that), that's not always entirely the case. *sighs to self* Draco and his ways...