Why Does My Attempts To Answer a Post Not Alert The Recipient?

OK, Not music or equip. related, but bothersome. Often I go to answer a post by typing the @ symbol, wherein, a list of people in the thread then show up. However, this is often incomplete and often does not show the intended recipient. I usually make sure the spelling is correct and whether the capital first letter is appropriate. Often it is a small letter which I then type in. I don't remember this problem many years ago as a frequent  poster. Anyone know? Thanks in advance..