There were a bunch of numbered Voyager novels that were missing, but I think most of those made it within the past couple of years.
I think most of these AWOL titles may have started making the digital transition a year or so ago when S&S started trying to clean up a lot of the older titles.
They've cleaned up the formatting and added fonts on older titles and added improved title pages for the titles that didn't have one or had a crappy one. The funny thing is, quite a few of the "improved" titles use the nicer, higher resolution title page as the cover as well, where the old versions actually had the correct cover image.
Two steps forward, one step back. Go figure.