Whenever I try using the landscape painting tool, it decides not to paint certain places, so I end up with super hard lines where the texture stopped painting. There's areas of my landscape that I can never repaint, no matter if I change the height, reset the construction kit, or anything.

What's going on with the landscape tool?

With the landscape dialogue ope, put your mouse over the cell with the hard edge. Press I to open the quad editor. You will note 4 lists of textures. Each quad, or 1/4th of a cell, can only have 7(?) textures in it. Delete the ones with the lowest percentage use to free up space.

Though I've not been able to, others note that you should never delete the first texture else it will cause irreparable black void shading to appear on the terrain.