There are several reasons I have come across throughout the years. One could be a bad low E string. The wrapping on the string could have come loose or not have physical properties to attract the magnetic field in the pickup. Another can be the magnet rod pole piece has lost it’s magnetic field or has become degaussed. The particular magnet could be bad and not cast the proper specifications of an Alnico magnet. The combination of aluminum, cobalt, nickel and other materials may have not been fully mixed when the material was in the molten stage of casting. In a few cases I have had magnets that wouldn’t stay magnetized after they where charged. This rarely happens but a few get through even though magnet suppliers have a high amount of quality control. The pickup will sound softer and less bright with a weaker magnet. Check to see if the string is resting properly in the nut and it’s at a lower angle from the nut to tuning key. Old and dirty strings will make it sound dull too.