Falling through floor glitch

I'm making a simple 2.5 D platformer, and I'm running into a problem where the Player object is falling through the floor. It only happens sometimes; it only happens above a certain height, and not predictably even then. The player object has a rigidbody and a box collider, and some floors are planes, and some are cubes, each with their own simple colliders. I've fallen through both the cubes and the planes, and it seems to happen at random, and extreme heights don't seem to be a guarantee of recreating the glitch. I'm controlling the Player Character with a simple C# script (I plan to add more features after I get this part functioning reliably):

using UnityEngine; using System.Collections; public class PlayerController : MonoBehaviour {

 public float moveSpeed, jumpForce;

 Rigidbody rb;
 float horizontal;

 void Start () {
     rb = GetComponent<Rigidbody> ();
 }
 
 void Update () {
     if (Input.GetKeyDown (KeyCode.UpArrow) || Input.GetKeyDown ("w")) {
         Jump ();
     }
     horizontal = Input.GetAxisRaw("Horizontal");
     Vector3 movement = new Vector3 (horizontal * moveSpeed, rb.velocity.y, 0f);
     rb.velocity = movement;
 }

 void Jump(){
     rb.velocity = new Vector3 (rb.velocity.x, 0f, 0f);
     rb.AddForce (Vector3.up * jumpForce);
 }

}

Can anyone explain why this is happening so I can fix it?