I've been helping someone with a tiny baby - 36 hours old when I saw her. Baby rooting well, latched fine, but ... just sitting there, still.
There isn't a magic 'damnit start sucking now' button somewhere is there?
And yes, she will suck on a knuckle, so it's unlikely to be that she can't suck. Mums nipples are flattish, even retreating a bit, which may well be the problem.