It depends. Are we talking pistol or rifle? Benched or offhand? What distance, what conditions, what basic ballistic performance of the load?
Horizontal stringing in a benched rifle is often a result of failing to appreciate just how much a gusty wind can affect the flight of a bullet. Too, bad bench technique can cause all kinds of problems at the target.
For ofhand pistols, there are all sorts of holding errors that can push bullet impacts left or right (though usually not both).
We need a little more information before a good answer can be found. For example, has this particular combo shot well for you in the past and has suddenly gone sour? Gradually? Or is this a new combo that you haven't yet fully tested?