The model of the atom that is missing from the set is the Thomson’s model. J. J. Thomson portrayed his atom model to look like a plum pudding. In his model, he described that an atom is composed mainly of electrons.
1). Wind more turns of wire in the coil around the core.
2). Increase the electrical current flowing in the wire.
(You can do that by using a battery or power supply with a little more voltage.)
3). If the core of the electromagnet is anything else but pure iron,
take it out, throw it away, and replace it with a core of pure iron.