Imagine how the current flows around the coil. Considering that the magnets remain in their fixed position, when the coil has turned half a turn the current is now flowing in opposite directions in relation to the magnets, even though the direction of the current doesn't change in relation to the coil itself. This causes the forces to change and makes the coil want to turn the other way, but when it does and has gone back to its original orientation after half a turn, the current is once again flowing in opposite directions. So it becomes an endless cycle of the coil making opposing half-turns and therefore it doesn't spin properly.