andi06 hit the nail on the head. No matter how small the turntable, some part of its mesh must be under the red and green arrows. I ran into this when I was making some really small broad gauge turntables. The answer is to place a 10 meter square part a little below ground level (with a hole in it for the pit). You won't see it but it will make the arrows work.
Ben
P.S. Sometime the loco itself gets in the way - just keep trying.