Ohh Eck!
Give me the easy ones why don't you
Well the truth is that without being there I can't give an accurate answer, but there are few things that can cause it.
When you say "you are being fooled", do you mean you are getting a positive reading and if so what numbers?
All detectors can and do get fooled by large or circular pieces of iron this is down (allegedly) to the wrap around effect where the signal is so intensely conductive that it crosses over the ferrous/non ferrous barrier.
Now add to this the improved conductivity gained in moist soil and this could explain some of the problems your having.
Now here's one thing I have noticed which could be the same as you, especially as you mention "clay". The Polden Hills where most of my detecting takes place is an outcrop of Lias rock this is part of a band of Lias that stretches across the country and outcrops in certain places (worcester is one place, so Hereford could be another) and this is, in wetter areas, covered in a lias clay that is high in aluminium and iron (lias itself is an iron bearing rock) and I have found that at the onset of wet weather some fields become almost undetectable for several weeks, but by February this effect has dissipated.
As to what is actually happening or why it lessens after months of rain I can't be certain but believe it's something to do with the machine almost being set with a hair trigger due to the GB and the iron being the better or easier signal tips the machine over the edge. So instead of blanking over iron many pieces start giving a positive signal.
I know this doesn't help in solving the problem but rest assured your not alone
