Hi epmaniac. The following isn't quite clear to me....
iam now noticing that, results for many terms are replaced by subdomain ... strange thing is that main domain pages are in collapsible + sign in subdomain result, which i can see
From your description, it sounds like your pages are being use as content in a collapsible div that's appearing on a subdomain, and that your pages are being filtered out as dupe content. That's a gut level guess.
It's not clear whether this is your subdomain, or a subdomain on another site.
If another site, it could well be that your site has been scraped and used as semi-hidden content on a scraper subdomain. Do some exact (quoted) searches for, say, 5-word text strings on Google. Also, search Google for longer strings, of say a few sentences, unquoted. Additionally, check your missing pages on Copyscape.
None of the methods is 100% dependable in locating dupe content, but together they should turn up with something.
Additionally, try searches for your pages... both on the exact strings and on terms you used to rank for... and append &filter=0
to the end of the url (in the address bar) of the Google results page. This string turns off Google's dupe filter, and can be helpful in determining whether you're still in the index but have been filter as duplicate content.