Some photos are blurry

I run a photography website and have been resizing my photo files to about 900×600. This has worked great for me up until now, but I was just looking through my gallery and while most of the photos come up clear and great, about 4 of them come up really blurry. I tried re-uploading them after checking their size, but it’s not doing anything.

Anyone else have a problem with this? It’s not the thumbnail view, it’s the actual view of the photo when you click on it. Here is the link, and thanks in advance for any help you can give me!

https://gracebalticphotography.com/?gallery_page=slideshow&pp_gallery_id=1379085547