Backlinks Not Crawling

[nagashree]

Hi, 

Please help me, I have 3000 Backlinks on my site, but every time Google bot crawls only 1000 or 800 links on each crawl only Why?

Some backlink sources:

• block Googlebot
  
• use
  
  Code:

  nofollow
  
  
• have low crawlability
  
• are not indexed themselves
  

These backlinks will not be crawled.