Hi...
I want reason y some of my sites pages are not indexed?

sometimes, some pages of your site are really hard to indexed. google has many data centers and so some pages take time before it get indexed.

continue on building back links to this pages

It really depends on a lot of things, most importantly the content on the page. ( Since we're talking about pages, I'm assuming you're using html, or something that renders it, and not Flash, pdfs, or something else. )

Has your site been crawled by Google? That's completely different from being indexed. If there are links to your site, it's almost certainly been crawled. If GoogleBot hasn't visited yet, you need some links pointing to your site from others they know about; then it will take a few days to be crawled, and assuming there's nothing wrong, to push your site through their index.

If your site has been crawled more than three or four days ago, we would need to look at the site to give a decent guess.

It depends on how old your website is. New sites may take a long painful time before its properly indexed. As mentioned in above posts it helps to find link partners and register your site in directories. My experience is that Google index new sites faster than MSN and Yahoo if you use sitemaps. Check in the Google webmaster resources if there are any error reports.

I can Share a good way create a site map and submit it to google and you can see your site getting indexed with few days. login to google if you dont have account open a account with google and in webmaster tools submit your site name. check after 1 day if it should any errors. And after few days it will give you details as which pages are indexed....