Why did Dobereiner's system of classification fail

Why did Dobereiner's system of

classification fail? 

Answer: Major drawback of Deberneir’s classification was that it was valid only for a few groups of elements known during that time. 

He was able to identify three triads only. Also with more accurate measurements of atomic masses showed that the mid element of the triad did not really have the mean value of the sum of the other two elements of the triad. 

For elements of very low mass or very high mass, the law did not hold good. For example Flourine(F), Chlorine (Cl), Bromine(Br). Atomic mass of Cl is not

an arithmetic mean of atomic masses of F and Br