Clean both the arbor taper and the taper in the chuck. Use a solvent like Naptha that leaves no residue. DON'T use any abrasive. After cleaning carefully make sure no dust, lint or other foreign matter is on either surface. Don't touch either surface because even oil from your skin can cause problems with fit. Set up a support for the chuck so that you can use the spindle feed to force the arbor into the chuck with exact alignment. You shouldn't have to use enough force to deflect or bend anything--just enough to cause a good mating fit. Good luck, attaching a chuck to a tapered arbor can be a frustrating experience.