parting blade walks off the line I scribed

Blade starts on the line and is a sixteenth off by the time it gets serious. 1 inch mild. Carriage lock is on. I checked, because last time it wasn't.

Lock on doesn't mean the saddle isn't lifting. A dull blade, or one with no side clearance, will walk. Look at the blade from the end. If it's wider at the tip than the shank, it is steering.

Wider at the tip. I ground the sides until it wasn't. It stays on the line. I dislike how often the answer is the tool I already ground once.