I asked this to you because I had some single pics choosen from a gallery in some posts, and then the rest of the gallery below.
For me single pic works – written as you do, but now I removed size because in new version you can choose it in the options, I put size in shortcode only where I want it different, a vertical image e.g..
However, in before 2.x version I could exclude pics from the general gallery checking the option but at the same time I could display them as single pic, so they weren’t repeated in the gallery below. Now this doesn’t work anymore.
Now if you exclude an image from the gallery via the option you can’t even use it as a single pic, you see a message that the image is not available – that’s why I asked you, because could it be that you forgot to have them excluded in the manage gallery page.
If it’s the same case, check that option.
If it’s not, forget it.
In conclusion, for me, to use them as a single pic I had to removing the check from that option, that’s no good because now I have some images repeated (in post and gallery). It’s not a big problem, but I preferred as it was before. Now, the option to exclude some pics from gallery is completely useless.
I think it’s a developers inadvertence, I’ve open a topic for this, but it was ignored.
Anyway this is your topic, I said this only because could it be the same.