Why does this production line jam?

I've been trying to build the following somewhat complicated production line in Big Pharma, with colored arrows indicating intended flow of products:

As far as I can tell, the input/output ratios are correct, and there should be no ambiguity left in the intended directions of the belts. However, this setup persistently jams up around the leftmost loop -- deleting a few products will temporarily clear the jam but it jams back up immediately.

Is there some reason this production line keeps jamming up? Is it plausible that I could get it to flow in the intended manner without modifying it (or, say, by adding more fixed belts), or do I need to fundamentally rework my floorplan here?