Thumb Image 600×388 not sharp

Hi can help with this thumb-size image 600×388

URL: https://www.viilevent.com
As can see here in the section on sports photography the image is not sharp, especially using iPhone
The first image using the 600×388 size
The second image using the 900×600 ( original size )
I’m not sure why using the 600×388 and the other using the original size.
The one using 600×388 is the new post that I created using this theme.
The original size of the post has been created long back ago.
Here are the photos, you can see also them in the URL above, and can use iphone or mobile phone to see the first image not sharp.
https://app.box.com/s/lg4wupkluemiwdc9zt49xb193nhjx7xc

I can see in the Setting Thumbs not have that 600×388
https://app.box.com/s/6xahhwxvnnc29datfmikwjy2zquvzbr4
As I try to regenerate the image using the recommended plugin but it did not work.

Can you help how to use the original image instead to make it sharp, or the 600×300 crop can be sharp like the other thumb-size image?

Thank you

Yes I mentioned I have use that regenerate plugin
The problem is not the old image but the new one after I use this theme got this problem.

When I see the source code I notice that some use the image with 600X388 but the second photo using the original size image 900×600
That’s why I think look sharp

The other thumb size image being used in the big Grid but it’s sharp. Only in the block 5 have this issue, it use the 600×388 size but this size during generated it’s not sharp like others thumb

Yes the problem is in the Retina, I think when creating the thumbnail itself not sharp.
I replaced the thumbnail manually, so it’s now sharp in the Retina and Website.
I think for now, that’s the only solution

Thanks a lot Calin