Originally Posted by RachelBlackG

GA can be sometimes messed up. So basically according to Webmaster Tools' clicks there is nothing wrong, but there IS always something wrong, so you should check some things. Anyway here are my thoughts:

1) Google doesn't see your subpages as valuable since it indexes only 50% of your pages. It may slowly decreases the amount of indexed pages = your traffic slowly decreases.

2) Some of the linking sites received penalty and it slowly passes negative points to your domain as well. (links from blogs that are used only to link other sites become "adverts" over time, you should buy blog posts only from blogs that post content with no external links as well -> almost no one in adult do that). Site linking new (low quality) sites from every subpage is suspicious (it doesn't pay for trusted sources like wiki). So you should check if you have such links.

3) Is your site mobile friendly? You may start loosing mobile organic positions.

I have obtained links from two domains in last week and my organic traffic started to behave just like yours. These domains weren't 100% bad, but they had barely some content and only few linking domains with low (3-7) Trust on MajesticSEO. They had good PA/DA ratio, but the value of the link was questionable. My link was on homepage only, but in sidebar (not good position even tho everybody in adult use it). All these factors resulted in negative SEO value. When I removed the links my organic traffic immediately reacted and added +20% over last three days.

NOTE: When you know the date of change in your traffic, you can check with moz.com/google-algorithm-change