Whole code for movements for my Rigidbody-Sphere-Character that moves on a 45 degrees axis (don't mind the portuguese comments):

using UnityEngine;
using System.Collections;

public class Movement : MonoBehaviour
{
    #region Atributos

    private bool m_bGrounded;

    #endregion

    #region Propriedades

    public float Speed = 5;

    public float Jump = 3;

    public float Gravity = 10;

    #endregion

    #region Mtodos

    void Awake()
    {
        rigidbody.useGravity = false;
        m_bGrounded = false;
    }

    /// <summary>
    /// Atualizao do comportamento
    /// </summary>
    void FixedUpdate()
    {
        if (m_bGrounded)
        {
            //
            // Inicializa os valores fX e fZ baseando-se nos controles pressionados pelo jogador
            // aplicando a velocidade definida
            //
            float fX = Input.GetAxis("Horizontal") * Time.deltaTime * Speed * 100;
            float fZ = Input.GetAxis("Vertical") * Time.deltaTime * Speed * 100;

            //
            // Aplica as foras ao rigidbody do objeto
            //
            rigidbody.AddForce(fZ * Mathf.Cos(Mathf.PI / 4), 0, fZ * Mathf.Sin(Mathf.PI / 4));
            rigidbody.AddForce(fX * Mathf.Sin(3 * Mathf.PI / 4), 0, fX * Mathf.Cos(3 * Mathf.PI / 4));

            // Jump
            if (Input.GetButton("Jump"))
            {                
                rigidbody.velocity = new Vector3(rigidbody.velocity.x, CalculateJumpVerticalSpeed(), rigidbody.velocity.z);
            }

        }
        else
        {
            //
            // Inicializa os valores fX e fZ baseando-se nos controles pressionados pelo jogador
            // aplicando a velocidade definida
            //
            float fX = Input.GetAxis("Horizontal") * Time.deltaTime * Speed * 25;
            float fZ = Input.GetAxis("Vertical") * Time.deltaTime * Speed * 25;

            //
            // Aplica as foras ao rigidbody do objeto
            //
            rigidbody.AddForce(fZ * Mathf.Cos(Mathf.PI / 4), 0, fZ * Mathf.Sin(Mathf.PI / 4));
            rigidbody.AddForce(fX * Mathf.Sin(3 * Mathf.PI / 4), 0, fX * Mathf.Cos(3 * Mathf.PI / 4));
        }

        rigidbody.AddForce(new Vector3 (0, -Gravity * rigidbody.mass, 0));

        m_bGrounded = false;
    }

    /// <summary>
    /// Mtodo para quando o objeto est em contato
    /// </summary>
    /// <param name="collisionInfo">Informao das colises</param>
    void OnCollisionStay(Collision collisionInfo)
    {

        foreach (ContactPoint contactPoint in collisionInfo.contacts)
        {
            //
            // Se alguma delas for normal no eixo Z
            //
            if (contactPoint.normal == new Vector3(0, 1, 0))
            {
                //
                // Est no cho
                //
                m_bGrounded = true;
            }
        }
    }

    private float CalculateJumpVerticalSpeed()
    {
        // From the jump height and gravity we deduce the upwards speed 
        // for the character to reach at the apex.
        return Mathf.Sqrt(2 * Jump * Gravity);
    }

    #endregion
}

There are limits to how many keys the keyboard itself will actually keep track of, so it might just be the keyboard, and not Unity or your code.

(Game controllers don't seem to have this issue, only keyboards do. I don't know if the problem is the hardware, drivers, or OS, but its a common problem playing any game with a keyboard.)

I'm not sure if you are actually using enough keys to be hitting that limit. A good way to test it though is by trying other games using keyboard controls, and see if the control stops responding well when holding high numbers of keys at the same time.

Problem solved, you wouldn't believe if I said that the SPACE key was the problem. I still don't know why it won't work with SPACE but it works fine with any other key I tested.

Maybe SPACE is a kind of special key or just a bug... I don't know... I checked input options and tried either GetButton() and GetKey() for SPACE and it's definetly the key but it works fine with the character controller though...

If anyone out there knows the reason please answer here because I still want to jump with the SPACE key as it's a kind of universal key for jumping.

This is likely some kind of bug. I have the same issue with left ctrl also. If I drive my vehicle around and press forward and steer right (w and d) and press space or ctrl at the same, the key is not read. This works fine when playing e.g. BattleField on the same computer.

I think that the Unity guys should look into this. Something is not working properly.

/Ricky

Hello, any updates on this issue? How did you work around it? I really want to use the space button with the arrow keys :(

Thanks in advance! /Kweiko