Yes you will need a ceramic, ferrite or alnico bar magnet under or between the two rows of your pole pieces. Ideally one large magnet is used for this. The magnetic field of the large magnet magnetizes the pole pieces as long as they are in that field. If the large magnet is removed, the steel slugs or screws will hold a useless insignificant charge, which you are probably experiencing now. As a quick test, you could just wire the two ends of the pickup to an output jack, place your neo magnet on one of the pole pieces and test it next to a string.