Raising or Lowering Terrain is either invisible or not smooth

Hi, I am working on creating a landscape in Unity 2018.3 and as such I need to raise and lower the terrain. My problem is that for some reason I am unable to gradually raise the terrain, when I click once the terrain doesn't change at all (and yes I have checked my opacity), where as the 'brush' indicates that there is higher terrain at that spot yet it doesn't show. If I keep 'raising' the terrain eventually the terrain will just spike up and be visible but it looks terrible. If someone could explain why this is happening and how to fix it, that would be greatly appreciated. Thanks.