We know that the formula of impedance for $L-R$ circuit, $Z=\sqrt{R^2+X_L^2}$, here, $R=$resistance and $X_L=\omega L$, where, $L$ is the inductance when we insert the iron rod in inductor then the value of $L$ increased and then the value of impedance also increased, and due to constant voltage current will be decreased across the bulb and the glow of the bulb will decrease.