NodeJs require function not working - node.js

Question

I'm trying to include custom modules in my nodejs application. the problem that I'm facing is that whenever I try to include custom modules it says that it can't find that module.
Error: Cannot find module './queue/queue'
at Function.Module._resolveFilename (module.js:469:15)
at Function.Module._load (module.js:417:25)
at Module.require (module.js:497:17)
at require (internal/module.js:20:19)
at Object.<anonymous> (/home/contrive/tasweel/main.js :5:13)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
I googled to find some solution but none of them worked. I transfered my project in another pc and there it was not throwing the error. After this I thought maybe there is some problem with node. So I uninstalled it and installed it again but it is still throwing me the same error.
Node Version: v6.9.1
Npm Version: 3.10.8
The way I'm trying to include the module is as followed below
var express = require('express');
var firebase = require("firebase");
var Queue = require('firebase-queue');
var queue = require('./queue/index'); // custom module (throwing error can't find module)
var queue = require('./index'); // custom module (throwing error can't find module)
var app = express();
OS: Ubuntu 16.04 64 bit
Folder structure:
-node-modules
-queue
-index.js
-worker.js
-main.js // entry point for node application
Please tell me why am I facing this error. Thanks