The strength of a magnetic field depends on several factors. The grade of material and quality, how it is magnetized and type of magnetizer used, outside elements that can demagnetize or how random the magnetic domains are within the magnets. The magnetic path to the strings and through the coil. Alnico rod magnets used in Fender style pickups can become weaker with time, being close to alternating currents, high temperatures when grinding and striking with a ferrous object.