smokeythebear said:
I consider it that they are advancing at the rate of the winter weather. Basically, they only like to be in weather that is below-zero (or whatever arbitrary measurement of "cold enough"). So as more and more of Westeros descends into the thick of winter, they will slowly advance further south.
We also know they can't cross the wall yet, so this is definitely part of it, but I don't think it's too crazy for there to have been some "well where are they?" lines in the show to explain that. As it is, it just sort of looks like they've been doing the world's slowest march.
If anything, I think explaining the amount of time and the reasons they haven't attacked/crossed the wall yet would lend some credence to why the Citadel and others aren't believing them. If Jon Snow has been claiming the white walkers are coming for a long time, but still nobody else has seen them, it would make his story less believable.
From the way he tells it, they could arrive any moment, but it's been a long time since Hardhome and nobody else has seen any sign of them. It would help explain why people on the show are so skeptical.