SyncVar Hook not Updating after Client Joins

I'm having a bit of an issue. In my game, players control a little cube around that shoots, and I've recently implemented networking. Currently, I can't get my SyncVar for health to update as I want it to. I followed the recent tutorial on using the HLAPI and have since tried to transfer it into my game.

I'll try and run down my code, but there's quite a bit so I'll cut out all the non-essentials:

• OnStartLocalPlayer I set my player's health to its max.

• Whenever it takes damage i make sure the server does the calculation of the health (basically if isServer, then HP -= dam)

• Now my HP variable is a SyncVar with the hook updateBar that should update the UI (using isLocalPlayer to be sure).

Problem: Whenever I'm just the host, everything works fine. I can run around, take some damage and it shows. Now I want to connect a client, and nothing works. The hosts' HP bar (not the HP, the bar) is reset, and the client cannot take damage. The host can take damage but it doesn't register.

Code:

 [SyncVar(hook = "updateBar")]
 public float HP;
 
 public Image bar;
 public Image shield;
 public Text text;
 
 public override void OnStartLocalPlayer()
     {
         GetComponent<MeshRenderer>().material.color = Color.yellow;
 
         camera.enabled = true;
 
         HP = maxHP;
         shields = 0f;
     }
 
 public void damage(float dam)
     {
         if (!isServer)
         {
             return;
         }
 
         if (shields > 0f)
         {
             if(dam > shields)
             {
                 HP -= -(shields - dam);
                 shields = 0f;
             }
             else
             {
                 shields -= dam;
             }
             
         }
         else
         {
             HP -= dam;
         }
         //updateBar(HP);
         checkHealth();
     }
 
 void updateBar(float val)
     {
         if(!isLocalPlayer)
         {
             return;
         }
         
         text.text = "" + Mathf.Floor(val + shields);
         bar.rectTransform.anchoredPosition = Vector3.zero + Vector3.left * (100 - val / maxHP * 100);
     }
 
 private void checkHealth()
     {
         if (!isLocalPlayer)
         {
             return;
         }
         if (HP <= 0)
         {
             HP = 0f;
             //updateBar(HP);
             Camera.main.transform.parent = null;
             manager.GetComponent<GameManager>().playerDeath();
             Destroy(gameObject);
         }
         else if (HP > maxHP)
         {
             HP = maxHP;
         }
 
         if (shields <= 0f)
         {
             shields = 0f;
             GetComponent<Light>().enabled = false;
         }
     }