Why did the law of triads fail?

Explanation

i] A large number of similar elements could not be grouped into triads. For e.g., iron, manganese, nickel, cobalt, zinc, and copper, though similar, could not be placed in the triads.
ii] Dissimilar elements could also be grouped into triads.
iii] Döbereiner could identify only three triads from the elements known then. Several were left out. Li, Na, and K, Ca, Sr, Ba, and Cl, Br, and I were the only triads.