Sites with loads of iron, more difficult to hunt.
Not all detectors here equal either as far as locating things like relics, coins, etc.
Coil size is a player too.
Soil mineralization can be a player too.
One’s experience with detector they are using is a player too.
One must go slow, and approach spots on ground from multiple angles. Notice I said spots here. Meaning gridding a site as a whole is different here. Gridding a site won’t allow a detectorists to hits spots from as many different angles. Gridding is a way to begin though.
It is also helpful to gauge how dense and how big the ferrous is in a site per area wise. These areas where iron is bigger and or thicker, even more time and careful use of detector necessary.
Detector settings here especially disc setting is key to find nonferous ( more challenged) in sites with lots of iron.
Coils size bigger like stock can see things sometimes a smaller coil can’t alert on and the depth of such nonferrous is not considered out of range for smaller coil either.
Sweep speed and coil height management when moving coil around very important to squeeze everything any particular model is capable of locating.
I enjoy hunting such sites.
Requires patience though and due diligence.