Why does isGrounded sometimes return false even though the character is clearly grounded?

I am new to Unity and tried to add an airdash to my 3D platformer. The airdash uses the same button as the jump command however it can only be triggered midair. Sometimes (I don't know when and why) when the character is grounded he dashes on the ground instead of jumping. Printig out the value for isGrounded returns false eventhough the character has been grounded for several seconds.

My movement code:

         if (dashTimeLeft <= 0)
         {
             float ystore = moveDirection.y;
             moveDirection = transform.forward * Input.GetAxisRaw("Vertical") + transform.right * Input.GetAxisRaw("Horizontal");
             moveDirection.Normalize();
             moveDirection *= moveSpeed;
             moveDirection.y = ystore;
 
             if (charController.isGrounded)
             {
                 moveDirection.y = 0f;
                 if (Input.GetButtonDown("Jump"))
                 {
                     moveDirection.y = jumpForce;
                 }
             }
 
             moveDirection.y += Physics.gravity.y * Time.deltaTime * gravityScale;
 
             if (!charController.isGrounded && dashAvail)
             {
                 if (Input.GetButtonDown("Jump"))
                 {
                     if(Input.GetAxisRaw("Vertical") == 0 && Input.GetAxisRaw("Horizontal") == 0)
                     {
                         dashDirection = playerModel.transform.forward;
                     }
                     else
                     {
                         dashDirection = transform.forward * Input.GetAxisRaw("Vertical") + transform.right * Input.GetAxisRaw("Horizontal");
                     }
                     dashDirection.Normalize();
                     dashDirection *= dashForce;
                     dashTimeLeft = dashTime;
 
                     moveDirection = dashDirection;
 
                     print(charController.isGrounded);
                     Instantiate(dashEffect, transform.position + new Vector3(0f,1f,0f), Quaternion.Euler(playerModel.transform.rotation.eulerAngles.x, playerModel.transform.rotation.eulerAngles.y-180, playerModel.transform.rotation.eulerAngles.z));
 
                     dashAvail = false;
                 }
             }
 
             if(!dashAvail)
             {
                 dashAvail = charController.isGrounded;
             }
         }
         else
         {
             dashDirection = moveDirection;
             dashTimeLeft -= Time.deltaTime;
         }