I find altitude is the usual culprit when not pinging in the mountains. The overall size of the mountain area may be so small that the peaks are not high enough. You have to be up to obsidian level to find silver, usually.

Another way to test it is find a group of vertical, standing stones in the shape and size of a longship in a meadow. You have a about a 95%+ chance of treasure being there, an equipped wishbone should ping while inside the "ship" perimeter.