I'll just re-enforce all the good advice given so far.
Lets take one of my productive sites for example.
Back in 2012 i found 37 silver coins and 3 silver rings there, the easy ones. Went back numerous times after and got nothing of value and declared that site hunted out. I was using multi-conductive, man. sens and my Tornado coil at the time. Let me add, this historic site has been/still is pounded by detectorists. My guess is that the high density of deep nails and other junk there scared off any serious detectorists.
Fast forward to 2013 now using TTF/4TF, man. sens., Tornado coil and open mask. As indicated above, this site is heavily infested with deep corroded nails, more than one could imagine.
I managed to find 14 more silver coins and a bunch of Indians there at this 'hunted out' site. The vast majority 10 to 13 inches deep. Probably dug up a half dozen deep nails to every deep coin.
Unfortunately a smaller coil would not of found that deeper layer of coins. But in your case a smaller coil could certainly be capable of finding any coins within its depth range among the iron if persevered.
If you got the instinct, go with it and keep working that site by trying something different. And be prepared to dig a lot of that iron... No pain, No gain.
A lot of times that deep jumpy/goofy/ sometimes silver'ish ID is corroded iron but sometimes its a silver coin.
If you are after the really deep stuff, you will have a much better chance using open mask (or bare bones minimum discrim. pattern), dig everything deep no matter the ID and of course a larger coil.