The number of times that you've used the Dexnav overall (not just on one specific Pokemon) seems to affect how effectvive it is at finding Pokemon. When I first started playing, it took me quite a while to find special Pokemon, but by the time I got six badges (looking for a vulpix on Mt. Pyre), I was finding Pokemon with egg moves and perfect IVs after just a few encounters. Luck is also involved in this, as I still found several "normal" Vulpix mixed in with the "special" ones. I still have yet to find a shiny.
As far s "chaining" goes, it does seem to exist, but it's not nearly as reliable as the PokeRadar of yore was. Immediately after capturing or defeating a Pokemon, if you use the Dexnav for that same Pokemon, the odds of finding it are higher, and the chances of hidden Ability, egg moves, etc increases. They will laos start to run away faster. Failing to find anything, being spotted, or running from battle will all break the chain and make you start over, and the longer your chain is, the more skittish Pokemon get. I've yet to sucessfully chain more than five Pokemon in this manner. Again, I've not found a shiny, so I can't say if using the Dexnav improves the odds or not. It would make sense if it did, but it wouldn't surprise me if it didn't.