Why isn't my animation state changing properly?

This is the code I have in the controller:

 > using System.Collections; using
 > System.Collections.Generic; using
 > UnityEngine;
 > 
 > public class PlayerController :
 > MonoBehaviour {     public Rigidbody rb;
 >     public Animator anim;     public float
 > baseSpeed;     float sprintSpeed;     bool
 > isSprint;
 > 
 >     private void Start(){         rb =
 > GetComponent<Rigidbody> ();         anim =
 > GetComponent<Animator> ();
 >         sprintSpeed = 1;     }
 > 
 >     private void Update(){         if
 > (Input.GetKeyDown (KeyCode.LeftShift))
 > {             isSprint = true;             sprintSpeed =
 > 2;         } else {             isSprint = false;
 >             sprintSpeed = 1;         }     }
 > 
 >     private void FixedUpdate(){         float
 > moveHorizontal = Input.GetAxis
 > ("Horizontal");         float moveVertical =
 > Input.GetAxis ("Vertical");         Vector3
 > movement = new
 > Vector3(moveHorizontal,0,moveVertical);
 >         transform.rotation =
 > Quaternion.LookRotation(movement);
 > 
 >         if (isSprint) {             anim.SetBool
 > ("isSprint", true);         } else if
 > (rb.velocity.magnitude > 0 &
 > !isSprint) {             anim.SetBool
 > ("isSprint", false);
 >             anim.SetBool("isWalking",true);         }
 >         else {             anim.SetBool ("isSprint",
 > false);
 >             anim.SetBool("isWalking",false);
 >         }
 >                                  rb.AddForce (movement * baseSpeed * sprintSpeed);
 >         transform.Translate (movement *
 > Time.deltaTime, Space.World);     } }

When I press shift I'm expecting the character to switch to the sprint state. However it doesn't seem to be registering when I press shift, it never shifts state. Have I done something wrong?