I had the same trouble with Sovereigns on our Oregon beaches, they losing 1-2" depth there and inland too, plus they had really bad problems cancelling/distinguishing pulltabs, often calling them "iron". Our soil is just plain too nasty for many detectors, and especially so with most Garretts, Minelabs, Nautiluses, and high-end/high-gain Tesoros too. I felt sorry for one woman who was using a Cortez, it only got 3-4" inland while my cz was getting roughly 6" average. Whites, Teknetics, old Compasses, and Fishers do the best here overall, especially on our high magnetie/salt beaches. They had their design roots here, that's why.
I have found that the only VLF's that work reasonably well on Oregon beaches are the cz's. Whites PI's do even better. Whites MXT's though seem to LOVE pulltabs there!
LL