strange Boolean Union fail

Hi all,

I am creating jointed pieces for a 3d printed model which will clip together. I am trying to boolean union the final pieces at the moment - each piece consists of 3 closed breps which I need to combine. There are several hundred pieces, but each is on its own branch so it isn't such a crazy boolean task. I am finding that they all work perfectly except for 8 pieces which return 'boolean union set is empty' error. When I bake these into rhino the pieces have perfectly aligned co-planar faces who's vertices are perfectly matched, but they will still not boolean even in rhino (unless I bump tolerance to 100mm!). An odd thing is that if I move one of the pieces away, and then move it back to its original position, then it booleans fine.

Does any one have any ideas about why these few pieces are not working? Its kind of odd seeing as the other 390 work fine! I've uploaded a baked piece which won't union in my rhino. Unfortunately the whole model is vast so can't upload the whole lot.

Many thanks in advance!

Jack