My image handler has suddenly stopped working

Starting this morning, it's kind of crap shoot as to whether the additional images will show or not.

I can add the additional images in admin as always, but sometimes they're not showing on the product page. I haven't made any db changes or anything else.

Here's a product page for example, in admin this actually has 2 additional images but they're not showing up.

Here's a product that I also added today, and the additional image is showing!

h-e-l-p