What is the condition in which the dominant allele is unable to express itself fully?

Co-dominance or incomplete/semi-dominance.

Co-dominance is a mix between the two traits, for example (simplified), in roses, white crossed with red make pink. This makes 3 distinct variations.

Semi-dominance is a little more complicated as multiple variations may occur but they can all closely resemble eachother, but with minor variences. Generally, a heterozygous trait will have traits similar to the two different types of homozygous trait (i.e. (simplified) AA, Aa, and aA will all look slightly different, but AA contains traits of both Aa and aA). For example, tonkinese cats, show various semi-dominant types.