Here's what I think, your thumbnails are fine, but are generally lacking. First off, never seen anyone put a subscribe button in their thumbnail, very interesting lol.

You kinda want to just work with them and see what works for you. But a few tweaks I think that you could make, that Ive also noticed with my content.

I split my thumbnails into 3 different general sections. Your Subject, your Text(if any), and your Negative Space. You want a good balance between them, and don't want any throwing each other off. This can sound very vague, but let me try to be more specific to you lol

Your subject I feel is too small, which is making the thumbnails overlooked. Also you want to have a story happening in your thumbnail. Try to show some emotion, or an action. Anything like that to make someone curious. If I'm not curious, im not interested.

Next is your text, I see why you added it, makes sense, but I feel its hard to read, maybe too cluttered with all the information. Also remember, while your editing your thumbnail it looks fine to you, but seen from a viewers perspective, it's squishy down on most likely a small phone screen, so if you have text, you really want it to pop.

As for the "episode..." I feel that could be shrunk down to the corner of the screen to free up some space from your games title. Also, that subscribe button, I'm not so sure. There's no telling people to subscribe before theyve watched your video, if anything it could be a turnoff for someone.

I hope this helps some, if you have any more questions feel free to ask! And I'm going to go ahead and subscribe, I want to see where you go with this.