You haven't mentioned how far you are from the transmitters, or if you've got a realistic line-of-site to them (or not).
It's possible the DB8 is overkill, and you're picking up a lot of multipath. If you're not too far from the transmitters, try something smaller (like a DB2 or DB4 from the same folks).
You've got it pointed at the towers, but does that take it through a brick wall and the neighbor's apartment? Can you point it out a window to face the transmitters?
Believe it or not, directing it towards the transmitters might not be your best option. If line-of-sight is poor, you might be better off facing AWAY from the transmitters and picking up the signal bouncing off a nearby building. Try creatively positioning the antenna in a few random places (preferably facing out the window) and see what happens.
Also, do check with the folks on avsforum.com; there are regional-threads and people from your area can probably advise you best.
Finally, it's easier to advise if we know the geography. What's the zip code? Nearest cross street or a nearby address? Something to give us an idea what we're dealing with...
16.5 feet in the Twilight Zone = 1 Rod Serling