The issue is, it worked on one post but didn’t work on the others
No, it probably didn’t.
If you try to share a link directly on Twitter, you’ll see that it doesn’t include a preview card like this one does: https://twitter.com/NelioSoft/status/1238268777902624768
That’s because, as I said, your website doesn’t include the meta tags required by Twitter.
Now, why do you think it did work in the past? Well, if I had to guess, that’s probably because our plugin published a tweet with an image. Something like this tweet: https://twitter.com/NelioSoft/status/1238283880500789249
As you can see, both tweets look similar, but they’re actually different. The former shows a “card” with the title and excerpt of the post, whereas the latter shares an image (which, in this case, is the same image we used as the shared post’s featured image).
Our plugin generates social messages randomly and, therefore, it’s not guaranteed that it’ll generate a tweet with an image attached. But (usually) all tweets include a link so, if you have the correct metas in your site, the preview card will always be available.
it seems to work just fine on Facebook and Instagram; so I don’t quite understand why this is an issue on Twitter only?
They use a different set of meta tags. Moreover, I think they’re able to pull the featured image and title even if their own meta tags are not in place…
I can try Yoast, but I don’t want to be adding more plugins to my website, making it slower, just for the sake of a Twitter image; if that makes sense?
It definitely does. But using an SEO plugin can definitely help and the performance impact it’ll have on your site is very small.