Is there a reason why Unity would be able to read one variable in a script and not another?

I have a single script attached to a gamemanager object in my game that has two sections: one as a GameManager that holds data between scenes and one as a Serializable section which converts the data from the GameManager to enable the Save function to save it.

If I play the game through from scene 1 - scene 3 and then save, the variables are all successfully read by my Save function (in a separate script). If, however, I load a previous game, I run into problems. The data is successfully loaded into the game and I can play it (ie. the GameManager.control.xxxx variables successfully use the loaded data), but when I go to save again, I can't because the game cannot find the Serializable section (PlayerData.current.xxxx). ie. Unity says object reference is not set to an object for the following line of the Save function:

    Debug.Log ("Saved Data: " + PlayerData.current.healthpotion);

This is despite the fact it's already accessed it to make the original save file. Is there any reason why a game wouldn't be able to see another section of the same script?

Main script

 using UnityEngine;
 using System.Collections;
 
 public class GameManager : MonoBehaviour {
 
     public static GameManager control;
 
     //recording names
     public string username;
 
     //setting up potion score
     public int hlthpotion;
     public int mnpotion;
 
 ....
 
 [System.Serializable]
 public class PlayerData {
     public static PlayerData current;
     public string UserName;
     public int healthpotion;
     public int manapotion;
     public bool redWizard;
     public bool purpleWizard;
     public bool blueWizard;
 
 
     public PlayerData () {
         UserName = GameManager.control.username;
         redWizard = GameManager.control.redwizard;
         blueWizard = GameManager.control.bluewizard;
         purpleWizard = GameManager.control.purplewizard;
         healthpotion = GameManager.control.hlthpotion;
         manapotion = GameManager.control.mnpotion;
 
     }
 }

Loading/Saving script

     using UnityEngine;
     using System.Collections;
     using System.Collections.Generic;
     using System.Runtime.Serialization.Formatters.Binary;
     using System.IO;
     
     public static class SaveLoad {
     
         public static List<PlayerData> savedGames = new List<PlayerData> ();
     
         public static void Save () {
             Debug.Log ("Original Data: " + GameManager.control.hlthpotion);
             Debug.Log ("Saved Data: " + PlayerData.current.healthpotion);
             PlayerData.current.healthpotion = GameManager.control.hlthpotion;
             PlayerData.current.manapotion = GameManager.control.mnpotion;
             Debug.Log ("Game To Be Saved");
             Debug.Log ("Original Data" + GameManager.control.hlthpotion);
             Debug.Log ("Saved Data" + PlayerData.current.healthpotion);
             SaveLoad.savedGames.Add (PlayerData.current);
             Debug.Log ("Game Added");
             BinaryFormatter bf = new BinaryFormatter ();
             FileStream file = File.Create (Application.persistentDataPath + "/playerInfo.dat");
             bf.Serialize (file, SaveLoad.savedGames);
             file.Close ();
             Debug.Log ("Game Saved " + PlayerData.current.UserName);
         }
 
      public static void Load() {
         if (File.Exists (Application.persistentDataPath + "/playerInfo.dat")) {
             Debug.Log ("Save File exists");
             BinaryFormatter bf = new BinaryFormatter ();
             FileStream file = File.Open (Application.persistentDataPath + "/playerInfo.dat", FileMode.Open);
 //            SaveLoad.savedGames = (List<PlayerData>)bf.Deserialize (file);
             List<PlayerData> playerDataList = (List<PlayerData>)bf.Deserialize (file);
             file.Close ();
             Debug.Log ("Save File Closed");
             foreach (PlayerData Pd in playerDataList) {    
                 GameManager.control.username = Pd.UserName;
                 Debug.Log ("Username matched");
                 GameManager.control.redwizard = Pd.redWizard;
                 Debug.Log ("Red matched");
                 GameManager.control.bluewizard = Pd.blueWizard;
                 Debug.Log ("Blue matched");
                 GameManager.control.purplewizard = Pd.purpleWizard;
                 Debug.Log ("Purple matched");
                 GameManager.control.hlthpotion = Pd.healthpotion;
                 Debug.Log ("Health Potion = " + GameManager.control.hlthpotion);
                 GameManager.control.mnpotion = Pd.manapotion;
     //            Debug.Log ("Mana Potion: " + PlayerData.current.manapotion);
             }
 
         }
     }
 }