One and half year old AT&T HTC One X still doesn't get Android update (or it's faulty)

Not getting an OTA update is not unusual, and the factors you mentioned probably didn't cause this, such as the family plan and being out of the country for winter break (the last OTA started deploying May 2013).  There was just a glitch in the system that has happened to a number of folks, and likely not anything you did.

Be aware that OTA updates have to be installed in sequence.  Therefore, missing a particular update will basically break the OTA chain. Since you for some reason missed the (software version) 3.18 update, you will not receive the upcoming update to Android 4.2.2 (software version will probably be 5.XX) which is supposedly is going to start rolling out tomorrow (Jan. 29).  Note that I will be talking about the HTC software version number for here on out (not the Android version) as that makes the most sense to me.

Some (not all) folks have been able to get the update to work by doing a factory reset.  Of course, you will lose your personal data, so backup anything important accordingly.  If this works, you will be able to get the 3.18 OTA, and then (hopefully) the 5.X OTA when it rolls out.  This requires a couple things to happen correctly, so again its not a sure bet.

The other option (more surefire) is to install the 3.18 RUU (download the RUU file and run on a Windows PC).  Go to the following webpage and follow the instruction under "Manual system update instructions".  This also wipes the phone, so again backup anything you want to keep.  Then (hopefully) when the new 5.X update rolls out, you will get the notification.  Of course, there is always the chance that for some reason if you don't get the new 5.X OTA notification (similar to what happend to you for the 3.18 OTA).

http://www.htc.com/us/support/htc-one-x-att/news/

The last option I will suggest is to wait for the 5.X RUU.  This will likely get posted on HTC's website shortly after it start rolling out OTA.  When it does, download and run on a Windows PC.  RUU's typically do not need to be done in sequence.  So you should be able to go from software version 2.20 right to 5.X.  This is the most surefire way to get to the latest (5.X) software.  With the catch being that you need to wait until the 5.X RUU gets posted on the HTC website.

Thank you very much for the reply ACE. I've been caught up with school work and didn't think much about the update. I will have a look into this again.

I have tried the factory reset. Nothing happens when I click "Check updates". Does it probably take sometime after the factory reset for something to happen?

I shall try out the manual update now.

Thanks!