Also try using two tone ferrous (TTF) if the site is littered with iron, It won't null as much, and you will hear the good targets among the iron. As Angel said, also try cranking down the sensitivity. If the problem is iron, you don't need a high sensitivity to pull keepers from that site that other machines have missed -- the E-Trac will see many of them (assuming you have Trash set to high). Think of high sensitivity like high beams in fog at sites like that.
Another possibility is power lines. Are there power lines at this site? I have one site where I can't get a threshold due to overhead power lines and a nearby cell tower. The E-Trac is great on EMI, but can't get a threshold at this site on any channel in manual, and auto cranks it down pretty low. Still have pulled silvers at the site in manual so I know the machine is fine. Hunting without a threshold can be disconcerting, but it doesn't necessarily mean there are problems with the machine.
If in doubt, try your machine at a different site. Also, as soon as you turn on the machine and place it to the ground, you should hear a threshold for the first 3 seconds or so before the ground balancing kicks in, no matter what. It always does this, no matter how trashy or mineralized the site is. If it is doing this, then I would bet the machine is fine.