A note on deepness, and this is my personal observation, mileage may vary - It is not common [I hesitate to say rare] to find coins 8+" deep. Sure it happens and I know everyone can point me to youtube videos with ultra deep finds, but in reality, its not going to happen very often. When it does, it will most likely be a sketchy signal no matter what detector you are using, depending on the size of the coin of course, and its position in the ground. I found an 1835 large cent the other day for example, was only about 5" down but not a good signal it was weak and mixed with iron, something I ordinarily might not have dug, but I did since this was an area known to produce goodies. The coin was on edge was why it wasn't producing a good signal, so if you have a coin on edge at 10", odds are you will never know it was there.
It takes experience to find deep coins, normally for coin shooting you look
[listen] for high tones and a good repeating signal, but as they get further away from the coil [deeper], the detector struggles to produce a usable signal. It will be broken, faint, repeats sometimes others not, target id all over the place so forget about that, and a weak pinpoint, if one at all - So these are just the types of clues you would want if you are seeking deep, and its take time with the machine to get good at it. Often you will find yourself chasing phantoms, no target recovery. Often it will be iron "Iron, what ? I thought it was supposed to ignore iron ??" In theory yes, but not in reality. Iron can and does ring up good at times no matter what detector you are using, and because of its mass, you can find them deep.
Sometimes iron and a coin will be side by side deep, adding new challenges. But its been my experience most coins are 8" and less deep, those deeper an experienced detector has a challenge to find, those without experience almost no chance without regards to what machine you are using. My only advice really for anyone beginner or pro seeking deep is swing slow and listen. Forget about target id, forget about a mad rush to cover as much ground as possible in little time. If you swing fast I guarantee I would follow in your footsteps and show you what you missed - I have literally done this many times, I did it just the other day with holes all around from guys digging, and I pull out things they never knew were there because they were in a rush and were only digging signals that rang out like a cow bell.
And finally, be careful what you wish for, have you ever dug a hole 12" straight into the ground ? Some soil types it is all but impossible, hard baked clay, rocky soil for instance, especially with hot weather coming on.