If it doesn't lock on the item is junk,unless the item VDI is around 18's are nickles, 67 zinc pennies, 75-77 pennies, 79-80 dimes, 83-84 quarters.
If you want gold you need to dig junk. Gold usually locks onto a junk VDI #, junk usually jumps from number to number no postive lock.
When in doubt---DIG !!
If the detector is too noisey, turn down the threshold to as low as possible, to the point you can barely hear a threshold. Back off on the sensitivity.
The ONLY complaint I have about the MXT is detector balance. With the 6x9 DD coil, it's battery box heavy. You need to forcefully hold the detector coil just off the ground. The coil wants naturally to be 8 inches outward and up and the arm rest 5 inches below the elbow. Ahhh,soon you'll have Popeye forearms. The 6x9 DD coil is my coil of choice.
I have been studying this balance problem while digging. Looks like what might solve it, is if the battery box is moved forward or the handle backward.
The MXT is my favorite detector, love popping quarters out of the ground and it loves to find coins on edge.