WHen you say that "several" don't work, you mean you can get some full-sized NEF previews, but some don't ? Or do you mean that you can make it work with some formats, but those you mention don't work?
Which version are you using?
The first version of 3.0 had a bug in this area. The latest Beta of 3.0.1 has fixed this.
Try downloading the beta at http://www.iview-multimedia.com/beta/
and see if that fixes your problem.