Por que o Twitter não pré-visualiza as imagens do meu blog nos links?

  <b:if cond='data:blog.pageType == &quot;item&quot;'>
      <b:if cond='data:blog.postImageUrl'>
        <b:if cond='data:blog.metaDescription'>
          <meta content='summary' name='twitter:card'/>
          <meta content='@tuusuario' name='twitter:site'/>
          <meta content='@tuusuario' name='twitter:creator'/>
          <meta expr:content='data:blog.pageName' name='twitter:title'/>
          <meta expr:content='data:blog.metaDescription' name='twitter:description'/>
          <meta expr:content='data:blog.postImageUrl' name='twitter:image:src'/>
          <meta expr:content='data:blog.homepageUrl' name='twitter:domain'/>
        </b:if>
      </b:if>
</b:if>