I've done a number of digitizer replacements now across the spectrum of iPod, iPad, iPhone and other smartphone-type devices and they are all uniquely challenging in their own way.
But, like the last answer, I'd agree that a key step in a successful replacement is making sure that not only are the ribbon connectors seated properly and securely, but also that they STAY THAT WAY through the reassembly process. I just did an iPod 4G and I know what a properly seated ribbon feels like. Despite the challenge of seating form the underside of the panel that I couldn't remove entirely, I did obtain a click fit on the thin touch ribbon. However, as I reassembled the phone a very small portion of the thin black digitizer is forced to bend against the housing (by design) and this would cause the connector to come free just as it was set back in place . A small copper adhesive strip present on the original, but absent on the iPod I was fixing (previously fixed by someone less attentive) would probably have held it inand made this a 10-minute job instead of a frustrating hour (or so).
Mimicking what I saw in an instructional video, I made my own piece of adhesive backer using Reflectix metal duct tape. With this strip of tape in place, firmly holding the connector in position against the force of the ribbon conforming to the case during reassembly, I was able to get a successful test on the touch before reassembling.
I would venture to guess that many of these "non-funtioning touch screen" posts are a product of poorly positioned ribbon connectors OR more likely, connectors that were snapped in place properly, but came free during reassembly. I would also suggest adding a single Z accordion fold to the black digitizer ribbon, keeping it from resting under the digitizer back which will cause pressure on the display and create an irritating pressure hot spot. You'll know it when you see it.
Good luck, and be patient even as you back out the multitude in minuscule screws for the umpteenth time....