HashMap.get returns null

I have a class that is a holder class for a HashMap and I create an instance of it in my main class and use it throughout all my classes. I have checked that my object is being added to my map but at some point during runtime when I go to access a value from the hashmap it returns null. Any ideas?

// Own class public boolean isFrozen(UUID uuid)
{
    User user = userManager.get(uuid);

    return lastFrozenLocations.containsKey(uuid) || user.isFrozen();
}
// Another class  private static Map<UUID, User> users = new HashMap<UUID, User>();

public Collection<User> getAll()
{
    return users.values();
}

public User get(UUID uuid)
{
    return users.get(uuid);
}

public boolean has(UUID uuid)
{
    return users.containsKey(uuid);
}

public void add(User user)
{
    users.put(user.getUuid(), user);
}

public void remove(UUID uuid)
{
    users.remove(uuid);
}
//Again another class 

    UUID uuid = player.getUniqueId();

    if(userManager.has(uuid))
    {
        userManager.get(uuid).setOnline(true);
    }else new Load(player);
}