Seamlessly Repeating Bees...Or Fireflies?

I decided to do a little pattern exercise today, but it's become a little puzzle. For some reason I keep losing the lines when I try to make it into a swatch (at the bounding box >> crop part). If anyone knows why that's happening, I'd be most appreciative if you could help me out.

They're meant to be outlined like this:

But when I crop they end up looking like this (and they do repeat perfectly so at least that's fine):

So they look more like fireflies than bees (which is cute but I want to know why it's happening!)

edit: OK! I managed to get around the problem (sort of) my transporting the image from Illustrator to Photoshop (de-vectoring it in the process I guess) and making it a Photoshop pattern. Voila!