Why is not Mendel's second law always valid for two or more phenotypical traits of an individual?
The Mendel's second law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes during meiosis are segregate independently.
The Mendel's second law however isn't valid for phenotypical features conditioned by genes located in the same chromosome (genes under linkage), since these genes, known as linked genes, don't separate in meiosis (except for the phenomenon of crossing over).