I usually find that if the full-size preview doesn't appear after a couple of minutes (and it can take that long on larger images), then simply editing and saving (without making any changes) is enough to get it to generate it correctly (again, after up to a couple of minutes). I've not had to re-upload a photo yet to get the full-size preview to work, although that's not to say you'll never need to.
It is definitely a bug though -- it does feel like FAA needs to hire or contract someone (or more people) with a system testing background to improve the resilience of the site and knock a few of the more frequently-occurring issues on the head. There's little to nothing that doesn't ever work on here, but there's an annoying amount that *occasionally* doesn't work.