When sweeping over a target that deep in normal "search mode" you might only notice a null or change in threshold. Once you hit a change like that in threshold (null or tone change) then you need to center the coil over the target and wiggle it as fast and as short as possible. That will produce proper ID and audio on deep targets. Often a normal sweep over something that deep isn't going to sound off properly until you wiggle.
Also, you may have sensitivity too high. I've found too high can null out a target or at the very least produce a really bad ID/audo. Next time you stick it in the ground play with your sensitivity level. Change it and then wiggle short/fast over the target. I would be you'll be shocked to find that even if noon or 11PM on the dial seems stable, something as low as 2PM will hit hard on the target while something much higher (yet stable) will null it right out or at the very least destroy a good signal. Higher is NOT better or deeper, despite what people have learned on other machines. These Minelabs don't like riding the edge of stability if there is any kind of mineral content in the soil or sand. When that's the case something much lower gives much more depth. Try it.