Windows mail inbox has blue + when replies come in

[Louise123]

Hi - I am new to this. When a Windows Mail email arrives, my lap-top has suddenly started putting a blue + by messages when the heading is the same as a heading already in use, instead of listing the new message separately. I have to click on the blue + and then the new message appears. The trouble is that a new message may have a heading similar to one used ages ago and therefore I cannot find the new message. Can anyone please tell me how to restore the old system whereby I can see every new message that comes in?! Thanks! Louise123