Why is not Mendel's second law always valid for two or more phenotypical traits of an individual?
Mendel's second law, or the law of the independent assortment, is valid for genes located in dissimilar chromosomes. These genes during meiosis segregate independently.
Mendel's second law though is not valid for phenotypical features conditioned by genes located in the similar chromosome (genes under linkage), since these genes, called as linked genes, do not separate in meiosis (except for the phenomenon of crossing over).
Image Diversity: genetic linkage