Animation doesn't play all the way through unless holding the key down

I am new to Unity and I am working on my first 2D platformer.. my player is using the CharacterController2D from here:

https://github.com/prime31/CharacterController2D

I have various animations, like Idle, Walk, Jump, Kick, Punch, etc.. and for the most part they're working just fine. However, the Kick animation consists of 2 sprites (StartKick and EndKick). When I press the key to kick, it only plays the animation half way (to StartKick)... I need to hold the key down to make it play all the way. This is unsatisfactory; I would like to just tap the key and make the kick work all the way without having to hold the key down.

What can I do?

EDIT

By the way, here's my Update method in a script attached to the player (which makes use of the CharacterController2D that I mentioned above:

 public void Update()
 {
     // grab our current velocity to use as a base for all calculations
     velocity = controller.velocity;
 
     if (controller.isGrounded)
     {
         velocity.y = 0;
     }
 
     if (Input.GetKey(KeyCode.D))
     {
         //some logic
     }
     
     // lots of else ifs here... (removed to make it clearer), then for kick:
     
     else if (Input.GetKey(KeyCode.E))
     {
         normalizedHorizontalSpeed = 0;
 
         if (controller.isGrounded)
         {
             animator.Play(Constants.AnimationStateHashes.Rick.Kick);
         }
     }
     else
     {
         normalizedHorizontalSpeed = 0;
 
         if (controller.isGrounded)
         {
             animator.Play(Constants.AnimationStateHashes.Rick.Idle);
         }
     }
 
     // we can only jump whilst grounded
     if (controller.isGrounded && Input.GetKeyDown(KeyCode.W))
     {
         velocity.y = Mathf.Sqrt(2f * jumpHeight * -gravity);
         animator.Play(Constants.AnimationStateHashes.Rick.JumpOrBlock);
     }
 
     // apply horizontal speed smoothing it
     var smoothedMovementFactor = controller.isGrounded ? groundDamping : inAirDamping; // how fast do we change direction?
     velocity.x = Mathf.Lerp(velocity.x, normalizedHorizontalSpeed * runSpeed, Time.deltaTime * smoothedMovementFactor);
 
     // apply gravity before moving
     velocity.y += gravity * Time.deltaTime;
 
     controller.move(velocity * Time.deltaTime);
 }