I have made a huge factory to automatize labs, and in doing so have encountered multiple times a bug with U-turn belts.
Since a picture is worth a thousand words, here you go:

As you can see, the U-turn at the top is stuck. The problem is, its totally random. It can work without problem for 30mins, or get stuck after a few minutes only.
Since I used a lot of U-turn in my factory, I was forced to often run all around to gather the materials on the ground and unstuck the belts.
I found a way around by simply making the turns wider, to avoid any possibility of U-turn.
Here another picture of 3 cases where I got the bug, and how I have fixed it:
