In this aromatic compound, the lone of N9 is participate in resonance with the compound and complete the aromatic system. So this lone pair is delocalised and can not gain proton (H⁺)

When an atom is already part of $$\pi $$ bond then the lone pair of the atom can not participate in resonance. That lone pair is called localised lone pair.

Here in N1, N3 and N7 all the lone pairs are localised. As those lone pair do not participate in resonance so it can gain proton (H⁺) so those lone pair will increase the basicity of the compound.