When atoms lose more than one electron, the ionization energy to remove the second electron is always more than the ionization energy to remove the first. similarly, the ionization energy to remove the third electron is more than the second and so on. however, the increases in ionization energy upon the removal of subsequent electrons is not necessarily uniform?

The increase in the ionization energy upon the removal of the subsequent electrons are not necessarily uniform because the affinity of the electrons to the atom in every subshell is different causing to a difference in energy needed to remove these electrons. Each and every subshell in an atom is attached or attracted to the nucleus at different degrees or amounts. As a result, the energy that is required to remove the electrons in every subshell would be different although it increases, it would not be uniform.