WordPress won't resize images

On Dreamhost my WordPress blogs all resize uploaded images for me normally. But one I have on a private server will not resize uploaded images. When I want to insert them into a post all the sizes except full size are grayed out.

I checked permissions and the uploads folder is 777, as it should be I think. Its ownership (a numerical value) is different from other folders though, and I cannot change it. I don’t think that’s the issue anyway.

I even copies all the files in the ‘js’ folder from a working blog over to the private server, but the problem persists.

Any help deeply appreciated!