Steam overlay not showing up [Unity]

Hi, I can't get Steam overlay to show up. Once I execute the code bellow, both StartPurchase() and PayForPurchase() return the expected results but the Steam overlay still does not show up. In my Unity project I have a GameObject with the SteamManager script attached which triggers SteamAPI.Init(). I have another script which triggers PlayFabClientAPI.LoginWithSteam() and that one also returns the expected result. This is my code:

using PlayFab;
using PlayFab.ClientModels;
using Steamworks;
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;


public class PurchaseTest : MonoBehaviour {


    protected Callback<MicroTxnAuthorizationResponse_t> m_MicroTxnAuthorizationResponse;
    string orderID = "";
    public Text text;


    // Use this for initialization
    void Start()
    {
        if (SteamManager.Initialized)
        {
            m_MicroTxnAuthorizationResponse = Callback<MicroTxnAuthorizationResponse_t>.Create(OnMicroTxnAuthorizationResponse);
        }
    }


    public void StartPurchaseProcess()
    {
        PlayFabClientAPI.StartPurchase(new StartPurchaseRequest()
        {
            CatalogVersion = "myCatalog",
            Items = new List<ItemPurchaseRequest>()
          {
            new ItemPurchaseRequest()
            {
              ItemId = "sword",
              Quantity = 1,
              Annotation = "Purchased via in-game store"
            }
        }
        }, result =>
        {
            text.text += Environment.NewLine + "StartPurchase: " + result.OrderId;
            Debug.Log("StartPurchase: " + result);
            orderID = result.OrderId;
            PurchaseOnSteam(orderID);
        }, error =>
        {
            text.text += Environment.NewLine + "StartPurchase FAILED";
            Debug.Log("StartPurchase FAILED: " + error.ErrorMessage);
        });
    }




    private void PurchaseOnSteam(string orderId)
    {
        PlayFabClientAPI.PayForPurchase(new PayForPurchaseRequest()
        {
            OrderId = orderId,
            ProviderName = "Steam",
            Currency = "RM"
        }, result => {
            text.text += Environment.NewLine + "Purchase on steam";
            Debug.Log("PurchaseOnSteam Success: " + result);
        }, error => {
            text.text += Environment.NewLine + "SENT TO STEAM FAILED";
            Debug.Log("PurchaseOnSteam Failed: " + error.ErrorMessage);
        });
    }




    private void OnMicroTxnAuthorizationResponse(MicroTxnAuthorizationResponse_t pCallback)
    {
        if (pCallback.m_bAuthorized == 1)
        {
            text.text += Environment.NewLine + "Authorized Payment: " + pCallback.m_ulOrderID.ToString();


            Debug.Log("OnSteamApproval authorized!");
            ConfirmPurchase(orderID);
        }
        else
        {
            text.text += Environment.NewLine + "Failed to authorize payment";
            // Player didn't authorize the payment
            Debug.Log("Failed to authorize payment");
        }
    }




    private void ConfirmPurchase(string orderId)
    {
        PlayFabClientAPI.ConfirmPurchase(new ConfirmPurchaseRequest()
        {
            OrderId = orderId
        }, result =>
        {
            text.text += Environment.NewLine + "CONFIRMED PURCHASE";
            Debug.Log("Final ConfirmPurchase: " + result);
        }, error =>
        {
            text.text += Environment.NewLine + "FAILED: " + error.Error;
            text.text += Environment.NewLine + error.ErrorMessage;
            Debug.Log("Failed ConfirmPurchase: " + error.ErrorMessage);
        });
    }


}

What am I doing wrong? Hope someone can help me out with this.

Thanks!