Try running your machine in "Loc-Trac" vs using "Auto-Trac" for a piece and see how your machine responds. I know the experts will chime in and tell you that auto GB is the way to go, but I've actually done better and found more coins with auto disabled. Coincidence? I'm not sure, but I know the results and that's repeatedly hitting the same 10'x75' trail leading from a 1700's cabin remains to the barn.
Just a wag here and I mean a complete WAG, I think the auto GB gets overwhelmed trying to differentiate good targets, iron halo and ground mineralization in its brain. Getting a good GB in target free ground and then searching takes one possible issue out of the mix.
I really don't think depth is a problem with the Spectra series at all. At least not in my soil/location. Not trying to poo poo your problem...I've been there at one point. Try Loc-Trac and see what happens.