This is just a guess but are they smaller in size? According to the update list Ruben and team are working on selling photo's that are ready sized for the web. If your new uploads are small in resolution it may have something to do with that.
No, they are all 300dpi, though some may have been cropped a little. I have posted these same images to other sites that require images to be at least 4 megapixels, so size should not be an issue here.