If I add a motor to a block with a single metal piece attached it will turn smoothly. But if I add another motor and metal piece on the side of the blocks it struggles to turn! Observe the effect in this demo I made. The motors seem to run out of energy, some hardly turning at all! For reference there is a single motor at the front which turns as expected.
They struggle because they're hitting the other pieces. Motors are bigger than they look. You have to leave plenty of space around them for smooth operation.
Look closely at them and they struggle even when there is lots of room to move. Note the straight pipe on the back motor. I did some more tests and found they work normally if you leave a one block gap between them. None of the motors should touch each other in my demo if they rotated at the normal speed.
What @cimarronline is referring to is that the motors themselves extend slightly beyond the width of a block at their widest diameter, so they will stop rotating if something is next to them. See this demo:
