I cannot hear the distinction between certain sets of vowel sounds. Normally the words in each of these sets (and of several others) all sound identical to me: Don, Dawn; marry, merry, Mary; ah, awe; cot, caught; ferry, fairy. If the speaker's accent heightens the differences between them I might be able to tell them apart, but cannot tell which word is intended by which of the different pronunciations. I can do that only from context.
This may not be uncommon in American English; maps of regional variations in pronunciation suggest that about half of the country pronounce Don and Dawn the same way. This would explain why I hear those names the same way, but only if I lived in that part of the country.
What is this trait called? I wish I knew.