Explain why a person with an allele for a particular trait may not have a phenotype that shows that trait.

This is because maybe that allele for the trait is recessive and the person is heterozygous. 

If the person is heterozygous, it means that they have both the dominant and recessive allele for a trait. And by dominant and recessive, if dominant allele exists, no matter if recessive allele exists or not, the characteristic controlled by the dominant allele must be shown. This is their phenotype, which is the observable characteristics. 

For example, there are 2 alleles for controlling the eye color. Brown allele is dominant, while blue is recessive. Just like the question above, if the person has an allele for blue eye color, but they have the brown color allele too, their phenotype would be brown eye color, as the brown color is dominant over blue eye color. This matches the requirements of having the allele for a particular trait but not showing the phenotype of that.

This movement is also referred to as  glacial plucking