I've never had that problem before. I would guess it's a size issue, but you've probably opened large files before, right? Do the ones that won't open have anything in common, like are they from the same designer or a different file type? Did you try restarting your computer and opening it with fresh memory? I don't know if that trick still works anymore, but sometimes my computer gets really laggy when I've been scrapping for a while and restarting perks it right up.