You know, I had that happen with a batch of the White Labs Sweet Mead Yeast just the other day that I pitched into that leaky-lidded primary. There was no visible or olfactory change in the cider. It was until I decided to stir it a little (with a santized spoon) that it basically exploded in my face, fizz-wise. I mean, normally my cider doesn't develop much foam in the primary, but there was ZERO surface activity this time. It was, however, the first time I've used the yeast, so I wasn't sure what to expect.
It may be fermenting, just not visibly. You could try agitating it a little, or getting a SG reading, but you will risk getting nasties in there. I didn't care with mine, because it was such a small batch that I can stand to loose it. I'm sure somebody will give you better advice, but I wouldn't count it (the yeast) out yet.