Why does Google index some but not all?

I was wondering why Google index's pages about "blue widgets" from my site but does not index the page about "red widgets".

I've just started working on the site in question which has been about for about 5-6 years. This issue is applying to new pages - about two weeks old. I know it takes time, but why are some pages listed and some others not?

I would say all pages are equally optimized and it isn't a case of duplicate content, they are completely different products/info and I can't find any logical explanation.

Thanks,

 

Try using google sitemaps?

 

Should I do this although I have a sitemap link on the top of everypage?

Thanks for advice.

 

Have you tried linking the new pages from the home page?

 

Yeah - every page in the site that I want to be seen is on a king-kong of a nav bar on all pages (text obviously). There is no way on god's earth the pages could have been missed.

 

You also need some IBLs to the site so Google will index the whole thing.

 

Thanks exam, I'll keep working on it