With your focus back on the chuck, the first thing that should be done is to get a resistance reading from the chuck. To do so, connect a multimeter that can read ohms to your chuck wires and record your reading. Make sure your chuck has been off so the reading is accurate. With the reading, it should match the reading that is stated in the manual.
Another complication that the chuck could be facing is to do with the top plate. If the top plate is worn or thin, it can cause poor holding of your workpiece. Large dents, loose laminations, and smoothness of the top plate also come into play when examining the top plate. Additionally, the workpiece could be the issue. If the piece has poor surface condition, it could affect how well it is held onto the chuck.
While on the topic of the workpiece, the composition of the piece could also affect the holding ability. Steel suppliers occasionally change the amount of chromium used in the steel. The more chromium that is in the steel, the less magnetic the piece will be.