Magnets CAN get demagnetized over time It's not as easy to do as accidentally tearing a coil, but it is possible. AlNiCo is used because it retains magnetic charge for a long time - enough to be called a "permanent magnet". But it doesn't come out of the ground that way, and has to be charged. I've reversed the polarity of Alnico polepieces in pickups of mine by placing a strong neodymium over them. So, their "permanence" is simply a reflection of the last influence they are exposed to.
"Demagnetizer" tools abound. They work their dark magic by alternating magnetic charge in a way that results in a sort of "average" of north and south that adds up to zero or near zero. Leaning a guitar up against the power transformer side of an amp (or near an AC motor) can have a similar effect. The caveat is that, in the land of magnets, small distances mean a lot. That is, after all, why tweaking pickup height 1/16" this way or that has such a big impact. So this is not any sort of alarmist directive to NEVER lean your guitar up against your amp. Just a depiction of things that one doesn't think of mattering, that could have a cumulative impact you never suspected.
Use some light ferromagnetic object to sense whether the weak pickup is also pulling that object with less strength than the other pickup does.
Lastly, as I've had the humiliation of learning the hard way, it is possible to connect the two coils of a humbucker out of phase with each other, resulting in a substantially lower output.
