Seeing as how a discrete transponder code is issued for flight following, why can’t we see those flights using the service?
Sometimes you can.
Often times, that discrete code is in a pool of local codes that each facility has that aren’t really part of “the system.” Also, controllers will often enter an abbreviated identification for the aircraft into the system. For example, 123AB might be entered as “3AB.”
Also, even under conditions where it “works,” no origin/destination/arrival/departure information is ever available for VFR flights so you’ll only see the flight on the map.
It’s pretty difficult to find these flight tracks unless you know the N number and as dbaker points out a controller might abbreviate it. The flights I’ve been able catch are picked out from monitoring my scanner for pilots requesting flight following. I note the N number (or it’s facimile) and a few mins. later I can find the flight on FA.