i have changed template of the website for 2 times in 2 months . maybe this could effects on website . in addition : i don't change anything in headers.php or somewhere that could have influence on website indexing on google . like headers.php , categories , post links , tags and anything in urls . why there is only 2485 urls in web index from 8401 ?!
no its happens when u submit a sitemap , it includes tag page, archives and stuff which google doesn't find it important to index. so yea its a common issue happens with all caus google decides which content is worth indexing and which is not
if i duplicate contents and i change somethings ( like title , or links in content ) does google find it duplicated again ? or this is only for fully duplicated contents ?
It is good to change something in your similar pages, but I think you will have duplicate content even after these changes.
changing title and links alone may not be enough it would check your content in text form - they are more intelligent nowadays - algorithms they use on bots
Have you submitted a sitemap? This can also happen if the URLs that are not indexed are very very deep in the hierarchy and the Google crawler cannot get to them or doesn't seem them important enough to crawl. Build links to these pages, and also improve the internal linking of your site structure.
still i don't know why it's happening ! now i have 19000 submitted urls (with sitemap) in my dashboard and only 3000 urls in web index !