Indexed URLs reduced day by day

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  arathymol t p: Hi,
  
  My website has 121 webpages(121 URls). Till last week it was showing 120 pages has been indexed by google in the webmaster tools, since then the problem began that the number of URLs indexed by google shown is getting reduced day by day and today it is showing only 90 URLs are indexed. I want to know the reason for that and the best possible solution for the same.
   ;Thanks in advance
• 
  Jeewan Garg: + ;arathymol t p can you send me the iamge for the same to better understand it
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  Jim Munro: Hi +Jeewan Garg ;. Thank you for your participation here. Please do post requests for direct contact. Ask to post images here instead. A complete list of our permitted practices can be read at http://nseo.link.
• 
  Jeewan Garg: +Jim Munro ;This is our duty to serve participant. I am sorry , I was not aware of best practices.
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  vo thang: your question is very interesting.I also don't know exactly what is happening ? But I think your website have problem,Google is perhaps thinking your website is spam or using black hat seo.
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  Ankit Goyal: +arathymol t p first of all check the xml sitemap if it correctly placed or not. If it is right, then you can check the indexing via "site:" query in google. it will give you the exact number of uls that are indexed.
  
  Solution: You can fetch the urls in google websmaster which are not indexed.
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  PromozSEO: I believe you have a robots.txt file. Have you changed anything on that recently?
  Have you got any DNS errors recently?
  Is GWMT showing 404 or any other URL errors?
  Have you checked the 'Manual Action' section recently?
  Are all the pages have fresh, unique and good volume (min 750+ words) contents?
  Do you have contextual dofollow inner-links on pages (MUST)?
  Have you checked whether anyone is copying your contents or not? Use copyscape and file DMCA.
  
  +arathymol t p ;test all of these before doing anything.
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  Tony “Tiggerito” McCreath: I think I read that this was a known bug they are working on or have fixed (can't find the reference). Is your traffic still staying steady?
• 
  Tony “Tiggerito” McCreath: Got it: ;https://productforums.google.com/forum/#!topic/webmasters/1d_iJZhyUNM
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  Neeraj Pandey: hi +arathymol t p ,
  You should check your pages If they are not duplicate
  You should check your webmaster account if crawler is not crawling lesser pages
  You should check dns if server is performing ok
  Also completely agree with +PromozSEO ;
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  arathymol t p: Thank u all