There's a maximum dig depth. Chances are you're in the wrong spot if you dug down and didn't find it. Could be that it's not silver since the wishbone finds other things, or you've got a bug. You have to dig where the pinging is the quickest, not just anywhere while it's pinging.

Sounds like it may be a bug if you dug that much and still haven't found what's making the wishbone go off.

Maybe I've been lucky but I find silver pretty easy to find and dig up.