Even though our website is indexed by google, sitelinks are not getting generated. It's been almost 2 years now and still not sitelinks. Google webmaster tools has a lot of 404 errors. Are these 404 errors making it difficult for google to generate sitelinks ?
Yes, those 404 are making it difficult for Google to crawl you.
As Google starts crawling a site, the more trusted, valued, updated, etc. the site gets, the more Google crawls it again, maybe crawling it more often and maybe crawling it deeper into the site structure.
Those 404 are treated as really bad links to Google that decrease that value of your site. All those 404 include old/deprecated links from your sitemaps, broken links inside your pages or even old/broken/malformed links on other sites, pointing to yours, that Google thinks that are/were valuable sometime.
In my opinion, 18 months is way too much for those 404 to be there. Your site has been losing value all this time and it's probably been crawled maybe once or twice a month or a week, and only a little amount of pages.
The first step should be to FIX THEM as soon as possible.
- If the content is still in your site, 301 them to your actual site content URL.
- If they're no longer available, remove them from your sitemaps, ask the other sites to remove then from their sites, and remove them from your site!
While your 404s are been removed/crawled again, update your sitemaps and read some blogs around about it
PS: This is usually a slow process, so.. patience