When I attempt to use Clay Strips in sculpt mode, no matter what I do, there's only one place where it will add to, no matter whether I use the addition mode or subtraction mode. It will just take away from the model. Edit: This only happens on one specific object. All other objects are fine with it, but this one specifically doesn't like clay strips.
2 Answers
The normals are reversed on the object for which the Clay Strip brush is currently not working. To prove this, try sculpting on the same object with any other brush. You will find that the new choice of brush will also not work. With the offending object selected, go into Edit Mode, select all faces, and press Shift + n. Now your normals are pointing to the outside of your mesh instead of the inside. You should be able to sculpt on it now.
You can add a separate object like a cube and then add it to the original mesh with boolean modifier(union) where the Clay strip brush is glitching. Now you probably can sculpt on two opposite faces without add/removing from the other one. make sure to Detail flood fill in dyntopo(constant detail) remesh mode to make the cube sculptable.
The problem seems to arise when the object scale is too low, and the two faces/vertices are close together at this low scale.
You can also try to activate "Front Faces only" in brush settings