What's the CTR for the Export logos one compared to the others?
At face value there's nothing wrong with them from a quality level standpoint, and I like your style of background and outline, BUT from a first impressions stand point, only the top one is interesting to me.
I can tell by the titles that you're in the design field, but the 3 bottom ones do seem a bit generic. They speak business, but they don't specifically speak graphic design. I have a strong interest in graphic design so the top one I can see the adobe logos, so I can tell if i'm interested in exporting logos, that thumb will be of interest to me. But the other thumbnails, from a strangers perspective, I'd be like "some random dude talking about a topic... i'll scroll until the thumbnail looks more designery" which mean, the thumbnails LOOK good, but they don't speak graphic design. To me, they just tell me you know how to use photoshop to make a thumbnail. I'd focus on making the style of the thumbnails look a lot more design focused. And that's possibly with the use of different stock images.
To be fair, if you removed the background and actually used your studio as the background, depending on how your studio looks, your studio in the background could give more visual impression that you're a designer, talking about design topics. The keyboard stock image is too generic business, and the writing is too generic contract. I can tell they're about work, and about contracts, but they don't scream DESIGN contracts. As a designer, I'm more interested in contract talk that focuses on design, and I don't always read the title, so if the thumbnail doesn't scream to me DESIGN contract, I'll keep scrolling until I see a thumbnail that looks more focused on design.
So I guess the theme/style of the thumb could indeed be the issue. Worth experimenting to try some new approaches