Why isn't the American nickel magnetic? I never thought that modern American nickels actually contained nickel anymore. However, according to this wiki article, the coins actually do contain 25% nickel, the rest being copper. And yet, no US coin produced today is officially magnetic. Why is this alloy of nickel not attracted to a magnet? 

(and yes, I tried time and again to find and answer elsewhere online.)
There are many types of magnetic properties, including ferromagnetism, paramagnetism, diamagnetism, antiferromagnetism, ferrimagnetism, superparamagnetism, metamagnetism, spin glasses, and helimagnetism.  Many of these are too weak to cause any noticeable interaction with a magnet. The type of everyday magnetism you're thinking of, which nickel has, is ferromagnetism.

While nickel is ferromagnetic,  copper is not. As you said, the American nickel is currently 25% nickel and 75% copper.  According to this paper (from 1931!), in order for a nickel-copper alloy to be ferrogmagnetic, it must contain at least 56% nickel:


  ... 56 percent nickel is required before the alloy shows ferromagnetic properties at ordinary temperatures.


https://journals.aps.org/pr/abstract/10.1103/PhysRev.38.828 

E. H. Williams, Magnetic Properties of Copper-Nickel Alloys. Phys. Rev. 38, 828 (1931).