[Plugin: WordPress SEO by Yoast] index problem

Any body know why by google webmaster tool i have 297 links , but just 87 indexing and two days before where 160 indexing and two weeks ago where fully 297, its there a but or problem whith yoast xml sitemap that cause problems to indexing. because of that i have 60% less trafic. what ca be or what i can do ?

https://www.ads-software.com/extend/plugins/wordpress-seo/