It could be a number of things. Too high of sensitivity, too high of Iron Mask, running in All-Metal Mode and using the wrong tones, Recovery Setting, etc.
If you run too high of sensitivity, this could make rusty nails come in better making them sound like good targets, especially in wet ground conditions. Also, it will depend on what setting you are using with the sensitivity...manual or semi-auto. Manual sensitivity will make deep iron give better signals than semi-auto does, at least what I have seen out in the field. If the ground is saturated, this also will make the iron more conductive thus getting false signals and getting small rusty nails sounding like coins. I would try using a lower sensitivity around 22 semi-auto and see if that helps. One other thing...having Deep On and Fast Off will also sometimes increase the signal off of iron, just depends on the ground conditions. Fast On will make the targets a bit more choppy and after a little time logged onto your detector, you should be able to have an idea what is good and what isn't. Just takes time to learn what the machine is telling you.
If you are using Iron Mask and you are say set up around -15, then I would run in Ferrous Tones since then this will help distinguish what each target is. Low tone means something with higher ferrous content and higher tones would be lower ferrous and more conductive targets if I remember right. All-Metal is -16 Iron Mask and ferrous tones will have to be used to distinguish iron from conductive targets. Also, if the target has a high amount of ferrous content (if your using the digital numbers), then it is more likely iron. Going by sound and what the conductivity of the target is will help you with less iron targets. Just be careful when in high iron infested areas and use a smaller coil to seperate targets better. Not all higher ferrous targets will be iron. Some could be good targets next to iron which sometimes can be a little tricky to distinguish from but one thing to look for is if the target id starts to jump around a lot, that could be the signs of a coin next to iron. Again, it takes time out in the field to really get the hang of how to use ferrous tones and learn what targets to dig and not too. If you are using digital, 31 is usually iron but not all the time. You will have to go by sound and check from different angles to see if the target id fluctuates any. If so, it could be a good item near iron.
Another thing is to check your variability setting. The higher the setting the better, since this will give you more range on different signals you will recieve when hunting.
I hope some of this helps you out and if you need anymore advise or maybe some setting options, feel free to contact me by PM or ask someone here on the forum. Someone will help you out. Good Luck and HH to you.