That can be tough!
Only ideas.
If you can locate a similar charm you can test your detector on it. An item in the ground can be different , but similar ,and sometimes a good hit from having been there so long.
Don't worry about old horse manure ,but wear a dust mask if sifting that small area ,if soil can be removed. Or ,pull the nails after detecting them.
Sorting through nails can be a challenge when after smaller ,more subtle targets.
The shoe could be on edge or tilted. Should honk iron good though. If it was thrown by horse or human it could have run,skipped and hopped.
If near a fence it could run under into weeds ,it might have.
Detecting after a rain can help liven up targets.
I did not locate a fine gold chain in a barn ,or path to barn. Others had looked too.
Passed on searching for a diamond earring in a trashy area , but if I had it's mate to test on may have tried.
Located close to a bank and other businesses so would have been quite a social setting.
You tried. Don't feel bad for that. Try again. Overlap coil " path"at most accurate speed. After going one direction over site ,start over at a right angle and hit it again the opposite direction to grid it.