Recessive alleles sometimes remain unexpressed because

For a trait to show or express itself in an individual it must have two types of the same allele (for example P and p) showing on the chromosomes. The showing of both alleles on an offspring's chromosomes means that both parents have the same trait and that is why there is a chance for an allele to become dominant in other words to express itself in the offspring. However if only one allele is transferred in the genes of the offspring that means that the trait that the allele is responsible for, will remain unexpressed.