Well if they worked for you before, I can't see how it would be the material ...
Different riding position could make you need different shorts. Are you using the same saddle on both bikes? That could contribute too. But really, those kind of things I would think would give you chafing, not numbness. I'm going to guess you're on the right track with them being tight. Not so much that the tightness itself is giving you trouble, but that it's changing the way you sit on the saddle.