Normally, if I hear my good friend Shimon say a bracha, I answer Amen. Nonetheless, if it's during davening, and I hear him finish a bracha, it appears to be common practice to not say Amen.
In other words, (as far as I observed) during davening everyone exclusively says Amen to the brachos of the Chazzan, and to no one else's.
But why is this so? Shouldn't Amen normally be said on all brachos?