A couple things
First, are you sure that there actually were new posts on the thread? There's a bump button inside each thread that bumps the thread up to the top of the most recent posts without adding any new posts or sending any notifications. Basically all it does it take the last reply and change the timestamp. This is supposed to be used to bring more exposure to low traffic threads.
Now, if there are new posts and you're still not getting notifications you'll want to check your account preferences. Click on your name at the top right part of the screen or go to https://xbunker.nu/account/preferences - then check to make sure you don't have any boxes unchecked at the bottom for the alert types you want.
Finally, there is a list of watched threads. It's right at the top of the page toward the middle. Click on the part that says "watched" with the eyeball emoji next to it and you'll have a list of all the threads you've subscribed to.