Quick answer: Your blend tree is stuck because the Animator parameter it reads is not the one you are writing to. Verify the name, case, and type; make sure each motion slot holds a real clip; then add a damp time to SetFloat so the blend position can move.

A 1D blend tree that locks to its leftmost clip is one of the most frustrating Unity animation bugs, and it almost never points at the blend tree itself. The issue is typically upstream — a parameter mismatch, a missing clip in one of the motion slots, or an update mode that is silently eating your input every frame.

Confirm the parameter is actually changing

The blend tree works off a single float parameter (or two, for 2D trees). If that value never updates, the blend position is frozen. Before touching the tree, log the value that Unity actually sees on the Animator:

void Update() {
    float speed = controller.velocity.magnitude;
    animator.SetFloat("Speed", speed, 0.15f, Time.deltaTime);
    // Read it back to be sure it is landing
    Debug.Log(animator.GetFloat("Speed"));
}

If GetFloat returns zero even when speed is non-zero, the parameter name in the blend tree does not match. Open the Animator, double click the blend tree, and read the string in the Parameter dropdown. Compare it character by character with the literal in your script. The name is case sensitive, and a trailing space will silently fail.

Check every motion field for a missing clip

A blend tree with an empty Motion slot will render as if the blend position is clamped to whichever neighbor is assigned. Unity does not warn you about this at runtime — the blend math runs, it just blends between a clip and nothing. Open the tree and verify every row in the Motion list. If you recently renamed or moved a MotionClip asset, the reference will show up as “None (Motion)” with no red outline.

The same trap hits nested blend trees. A 2D tree that wraps four 1D children will inherit the empty slot problem from any child, and from the outside the bug looks like “the top-level parameter does not work.”

Add damping so the value can travel

If you call SetFloat without a damp time, the parameter snaps to the new value instantly. That is usually fine, but if the incoming value flickers around two thresholds — say, velocity of 0.49 one frame and 0.51 the next — the animator thrashes between clips and the resulting pose looks like it is stuck at one end. Pass a damp time of around one-tenth of a second:

// Smooth over 100ms using the current deltaTime
animator.SetFloat("Forward", targetForward, 0.1f, Time.deltaTime);

The damped version uses Unity’s internal critically damped spring, so the blend position eases toward the target without overshooting. This is also what the default locomotion templates ship with.

Match the Animator update mode to your input source

On the Animator component there is an Update Mode dropdown: Normal, Animate Physics, and Unscaled Time. If you set the Animator to Animate Physics but write parameters from Update, you will sometimes see the blend position lag by one FixedUpdate tick. In most games that is invisible, but on a short mechanic — a dodge, a quick aim — it looks like the blend failed to fire. Move the SetFloat calls into FixedUpdate when the Animator is in physics mode, or switch the Animator back to Normal if your character is kinematic.

Verify the blend tree thresholds

Blend trees will clamp the parameter to the range defined by the thresholds. If your parameter goes from 0 to 6 but every threshold is below 1, the tree will always sit at the last motion. Click “Compute Thresholds” in the blend tree header to rebuild them from the clip root motion, or enter them manually. For 2D trees, watch the threshold points on the graph — a collapsed cluster means one axis is dead.

“Nine times out of ten the blend tree works and a parameter further up the pipeline has the wrong name. Log the value on the frame before you log the animation problem.”

Related Issues

If your animations drift even after the blend is working, see Fix Godot AnimationTree blend not transitioning for a parallel debugging approach in another engine, and Fix Godot animation looping not working for looping edge cases that look similar on screen.

Tip: temporarily set damp time to 0 while debugging — it makes the true input value obvious on screen.