You mentioned the site is fairly new, do you have backlinks coming to the internal pages (articles pages)? If no, this can be the reason they are not appearing in Search results.

As mentioned article pages were fairly indexed and even appear on the 1st page of search results.

It was lately, when I started to do link building that it was not being indexed, though it is still on the 1st page of search results but it was not the articles itself that is being shown on search results instead the homepage.

Have you ever built a site before and watched it's presence grow in Google? The reason I ask is that it is common to do well at the very beginning and then see it fall away - I've called it a honeymoon period. Google seems to grant it just in case a new website really "catches fire."

More commonly, the site does not catch fire -- and soon the honeymoon is over. Then starts the gradual build up to a more stable indexing and ranking situation.

Hi Tedster,

Yes, I have a couple of sites that were fairly indexed from start til now. This issue only happened to this site.

I am thinking If I have done something wrong while promoting the site in forums, social networks, article submission etc...

Ah, that's very possible. Google has become rather persnickety about some kinds of backlinks lately.

I'd say you should work some other kinds of promotion now so you can attract a better variety of backlink types for your site -- especially those freely given links from within a content area of a page.