In RotH, Sheol first task, I only find 15 brains (4 in the main room, another 4 in the hallways after you got down the steps and 7 in the four heart rooms). From the symmetrie how the brains were found so far I fist thought, I missed one brain in the last heart room, but I didn't find it. I restored an older game and this time I marked every heart on a sketched area map. I only find 15.
Is there anything I have to do to make that 16th brain appear, or is there anything I should have done before?
I downloaded a walkthrough but it only says "find all brains".