With the right tool it's not that they're hard to detect...it's that people don't know how.
Low gain with high sensitivity - thats a fundamental worth repeating - Low gain with high sensitivity.
Go blasting around with your detector running way too hot - one will be lucky to hear anything but the largest studs.
Small coil can do better than larger coils, but I can and have hit studs with a 15x12 sef running ... again, very low gain and high sensitivity. (3rd time I've mentioned that- probably important)
Get your ground balance dead on too. That's more important than coil size.
Listen for very tiny short signals and verify it's not deep with your pinpointer. Tiny folded foil or gold stud...you'll greatly decrease the time to find it.
VDI? Ask for the matching earring and scan it. Otherwise, below nickel and into iron possibly.
Good luck!