Ok I've been saying "texlights" - I think the correct term is "lightmaps".
Basically I've got this bug where I shine a flashlight on a wall and it doesn't smoothly illuminate the wall, instead it just kind of 'stops' at a certain point (as defined by the surrounding geometry?) and then once the light gets over that stop, it illuminates the rest of the brush.
Ex.
Is there a way to solve this without reducing the size of the texture?