1019 is similar to other Duals. The cartridge slide is held in place by the silver arm grip located on top of the headshell. You take the cartridge slide, and put the end toward the arm in first. You then push cartridge slide straight back toward the arm pivot. You should then be able to raise front end of slide and it should go right into proper position and stay in place. You then take the little silver grip that you use to pick up arm and push that clockwise toward front of arm until it locks in place, about 90 degrees opposed to the head. This will engage the slide lock. Check the Dual 1019 manual found on this website. You will see how headshell is installed. It's easy, but you must be careful.
I just thought of something. In order to take the cartridge slide out, you rotate the silver arm grip toward the back of arm. The silver grip will rotate sideways, and the cartridge slide will come out if you push slide straight toward back of arm. If the silver arm grip is at 90 degrees to headshell, then the lock will keep the slide from going fully into position, and it will tend to fall out. You must have silver head grip rotated fully toward rear of arm for headshell to go into proper position, and this may be what is causing your headshell to not go into arm.
It may be difficult to understand what I am saying if you are inexperienced with these old Duals. I believe that the manual, located in Library under Dual, will have pictures to help.
And good luck from that visually-oriented old head remover,