Cannot change Mip Gen Settings of some imported textures

Hi,
Found out recently that some of my imported textures have just one Mip so the Mip Gen Settings drop-down many does not change from NoMipmaps option, while some other ones have 9 Mips.
I cannot find what causes this and what I should do in texture creation process before importing them to the engine to have multiple Mips.
Does anyone know about this matter?