How deep you can find a coin is predicated on several things (not exhaustive list)
1) The ground conditions (mineralization, acidity, moisture content, temperature of the ground at depth, etc)
2) How good of a ground balance you machine gets. All ground balances are not created equally. The ground balance on a Bounty Hunter is not going to be the same, or as good as the ground balance on a 3030. If your machine can't properly track the ground, it is going to muck with what your machine tells you. Good targets could sound bad. Bad targets could sound good.
3) The target itself - Conductivity, thickness, diameter, symmetry, and shape. Highly conductive targets produce a stronger induced response field (this decreases as frequency increases). A target with a large surface area will produce a stronger induced response field. A thick, highly conductive target with a wide surface area (like a silver dollar) will produce a much stronger response than a dime sized object made of the same material.
4) The position of the target in the ground. This is extremely important. If you find a 15" coin, you can bet that coin was lying as close to flat as can be. The more angled the coin, the less likely your detector is going to see it. It doesn't matter the machine you are using. All detectors are bound by the same math. If someone tells you they found a dime on edge at 12", they are either lying, or they picked up on something bigger, or shallower, and the dime was a secondary target the machine probably didn't even see. It doesn't matter what machine you are using - a deep coin on edge is for all intents and purposes - invisible.
How strong an induced field a target produces is based upon the flux passing through the surface area of the target. as the angle of a coin goes from flat (0 degrees) to on edge (90 degrees) you are progressing from maximum surface area to an absolute minimum surface area. The smaller the surface area, the weaker the induced field will be. The strength of a magnetic field is inversely proportional to distance (in this case, depth) the target is from the receive coil. At, say, 10 inches (25.4cm, 0.254m), the receive coil is trying to detect the response of a field that is effectively 61x weaker 10 inches away from the target. As a coin's angle increase, the field strength seen will be approximately cosine(coin angle)
5) Sweep speed. This is something that people tend to overlook. Most people I see have a really fast sweep speed. This is generally not good unless your detector has a really fast recovery delay. If you sweep too fast, your coil will be over and past a target before it finishes processing the signal, which will result in a clipped, masked, or entirely missed target. conversely, if you swing too slow, you are going to be seeing the same target multiple times. Faster swing speeds are generally better in highly mineralized ground, where as slower speeds work best in low mineral ground.
6) Swing consistency. You need to keep your coil on the ground. If you are U-arcing your swings, you only have a very small window in which your detector will detect its deepest (approximately 1 coil width). As you lift the coil away from the ground, it harder it is to detect an deep object. There is significant loss for every inch you raise your coil off the ground. (using the above example, bringing your coil 1 inch off the ground will turn that strength 1 signal to 0.76, 2 inches makes it 0.59, etc. This isn't much of an issue for shallow targets. It becomes a huge issue for deep targets.
7) Sweep direction. This is why many people grid. You could miss a target depending how you sweep across it. This isn't so much important for an approximately flat coin. It is symmetrical and will respond uniformly regardless of the direction you sweep across it. This changes as a coin's angle increases from flat to edge. The target is no longer symmetrical in all directions. It will sound different in one direction than it will in another. Those targets will also pinpoint with an offset either to the left, right, toe, or heal of the coil depending upon the angle of the coin and the direction you sweep. This is why it is important to pin point from two directions (it eliminates the pinpointing offset caused by the coin being at an angle). If you only pinpoint in one direction, you will be digging in the plug walls for your target. As you can imagine, the deeper the target, the more important this becomes.
8) How well you understand your detector. If you don't have a firm grasp on what your detector is telling you, you will have a real hard time finding the deep targets. If you run your detector too hot for a particular patch of ground, you are going to lose depth. You need to know when you are running too hot.
9) Your detector in general. Depth doesn't come from raw power. I can raise the output of my transmit coil from 10v to 30v. That is 3x the output. It doesn't get me 3x the depth. It may get me an inch or two depending upon the soil conditions. Depth comes from better coils and digital processing. If your coil doesn't have a good null, it is going to be noisy and will affect your depth. if your machine has average processing abilities, you are going to have a hard time pulling those very faint signals from everything the detector is processing. Skill can only take you so far. You could be a master using a low end detector. You could out hunt average or better than average users of better detectors. In the end, someone who is just as skilled as you are, using a high end detector is going to find things you do not.