The pinpoint feature on the Nox is automatically tuning when you turn it on and then automatically retuning as you swing.
That means if your coil is already against the ground, and you press pinpoint directly above a strong signal, or swing right away over a strong target, then it takes an even stronger signal to hear pinpoint audio. So, obviously, you don't want to start pinpoint above your target. If you press pinpoint with no metal present, then even the smallest signal will trigger the pinpoint audio.
This feature doesn't work so well when you have a strong trash signal near your intended target. You might not even be aware of that trash signal if you have iron discriminated out, or are running a cherry picking discrimination pattern. (The pinpoint feature doesn't care about your discrimination pattern.) This explains the erratic pinpointing behavior people report with the Nox.
So, to pinpoint the quieter good target you not only have to avoid pressing the pinpoint button over that stronger junk target, you also have to avoid swinging over it while you are trying to pinpoint your quieter signal. If you don't, it'll retune and you'll lose your quieter good target.