Is there a history of deeper finds in the area? The clay can many times be effective at stopping or vastly slowing the sinking of objects with time. I know this is an elementary question, but until we've established that there ARE objects deeper than you are finding them, all other discussions become somewhat acedemic.
One thing to try if you haven't alreay is to bury a known object in a clean area. One method I use is to use a 1" oak dowel about 14 inches long with inches marked off. I place a coin on ground, center the "punch" over it, and hammer it down inch by inch as I gauge the detector's response.
We don't need to concern ourselves with the "halo effect" for this test, we just want to see where the stock coil can just barely read the target, then to compare the new coil's performance. If it were me, I wouldn't be happy with less than 2 inches greater depth for the new coil, assuming the ground is as "friendly" as you think it is.
Hope this helps, but again we need to quantify whats going on. Are there deeper targets for certain? Driving a coin to the limits of the stock coil, is it easily readable with the new?
Let's see if we can't get this figured out!
DAS