All rope halters are not created equal.  Some are made of rope that is too soft for good control and contact with your horse.  Look for a halter that is made of a smaller diameter of rope and is stiffer for better contact with the horse.  You may feel that a halter like this is too rough for your horse, but the idea is that they learn from the pressure applied to the pressure points on the horses face.  The pressure points are along the poll causing the horse to lower his/her head and along the nose which encourages the horse to tuck the nose in and respond to pressure not to lean on it creating a more responsive horse.  There are also sensitive points along the cheek/jaw and under the chin.  It may take a few times for your horse to learn the it is easier to give or yeild to the pressure than to fight it and often this makes areas where the hair is rubbed away, but this is only temporary and resolves in a few days.  Traditional flat style halters don't work the pressure points and can actually encourage your horse to lean on the halter when leading or longing on the long line.  Horses can ignore tremendous amounts of steady pressure without yielding that the flat halter provides where the rope halter creates discomfort along the sensitive pressure points and the horse yields more readily.  A suggestion for the horse owner is the pull sharply and release in succession until the horse yields rather than one long pull will keep the horse from "leaning" on the halter.  I have successfully started unbroke horses under the saddle in just a rope halter for control of his/her head using this method during ground training.  It's easy to advance into a sidepull then bosal afterward because they already give to pressure.