Might be that the nori sheets aren't at the right moisture level as well. We live in a humid area so I have to put mine flat in a dry pan over low heat for a minute or so (sometimes less) per side and they get "crisp" again. Also, work fast. I found when I am rolling maki if I use a plastic sheet over my bamboo mat (maki-su is the right name I think) I end up with soft nori because I work slowly. If I just go without the plastic I fare a bit better.