Why don't all mutations in DNA produce changes in amino acids?

A mutation is a change in the genetic code within a cell. New genes arise due to the process of mutation. New phenotypes can be produced in the organism that inherits these changes.

Sometimes, a mutation does not produce a noticeable change in the organism. 

In a point mutation, a chemical change occurs in one nucleotide pair of a gene. If this change results in one nucleotide pair being switched for another, it can possibly still be translated into the same amino acid, causing a silent mutation. This nucleotide pair substitution will result in no harmful effect on the protein function due to the redundancy of the genetic code. Many of the amino acids in the codon chart have more than one triplet to form that amino acid. 

If you refer to the link below, you can see the codon chart. Find the symbol Val, which is the amino acid valine. The combinations of GUU, GUC, GUA and GUG all produce the amino acid valine. If a GUU triplet in the original DNA was changed to GUC, this is considered a point mutation. It is a silent mutation with no ill effects on the protein produced. The change still results in the insertion of valine into the growing polypeptide.

In another example, if a mutation occurs in a region where the exact amino acid sequence is not necessary for the proper functioning of the protein, this can result in a mutation with no deleterious effects.

To conclude, any change in the DNA is known as a mutation, but not every mutation is harmful or has deleterious effects on an organism.