You may want to revisit the old photos you say you took on the D3100. Maybe post one with the EXIF data.
A 10 stop ND filter, with an exposure of around ISO 100, f/16, is required to get to the 30 second range in moderate shade, and sometimes that comes up a few stops short. Something is amiss here. The D7100 should perform the same, given the same settings, so it too will need a 10 stop ND filter.