I found that as well. When I first bought nori, I had to toast it, now I just buy the toasted nori and it works much better. I find too that if you let it sit for a hour or so before you eat it (in the fridge, of course) it becomes less chewy.
To toast it, I just heated my oven to 350 F, then, when the oven was ready, I just slid my sheets of nori onto the grill. Left it on there for about 15-20 seconds, and it turned bright green. Then, it was ready. I took it out and dressed it as I would normally.
I hope this helps,