Sometimes things just have to smack me upside the head.......

[pippin_999]

Lizzie:
> This could have been part of the reason, but Neville really was an awful potions student too.
> 

Pippin:
I think that's the only reason. One of the things that smack *me* on the head on re-reading is that Snape says nothing at all nasty to Neville in that first class, and in fact blames Harry for Neville's mishap. We hear at the end of PS/SS that Neville's potions grade was abysmal, but there's no indication that Snape's hostility has anything to do with it. It's only in PoA that we hear that Neville regularly went to pieces in potions class, that it was his worst subject, and that Neville's great fear of Professor Snape made it worse. 

It sounds to me as though Snape only gradually became frustrated with Neville's lack of improvement.

Pippin