Some of my Cards work; others don't


I recently installed Yoast SEO on my site in hopes of implementing Twitter cards.

When I found that it didn’t work, I also installed a plugin called “Per page head,” and I used it to enter the Twitter card meta data.

I still have Yoast activated, but I have its Twitter cards option disabled.

Now I find that cards work for some post/pages, but not for others.

For instance, it works for this page:

But not for this one:

Regarding posts, it works for this one (or, sometimes it does):

But not for this one:

The Twitter card validator only tells me, “ERROR: Fetching the page failed because the request timed out.”

Also–I just checked and saw that one of my cards that previously showed up in the timeline has now disappeared, while a couple others are still there.

Any suggestions for what I should do differently to make sure Twitter cards work for all my posts and pages?