Why does my headsail furler halyard bind up?

That is not an uncommon problem, but unfortunately, it, the friction, can come from several sources.

I would start with the basics. If the sail gets a kink (bend) in it before it enters the slot that is a problem. You can have someone feeding the sail into the slot or they make a little keeper to guide the sail.Failing that helping: I would inspect the bolt rope (luff cord) of the sail and be sure it is the correct size for the slot. Wax or silicone it if it feels like there is a lot of friction when you move it manually in the slot. Don't be afraid to use WD40, I have with success for a long time. From what you say: the sail slides smoothly through its track (are you sure there isn't an obstruction up 10 feet or on the sail its self about 10 feet from the head that does not want to go into the track?). You also say the halyard does not bind by its self. Have you tried it under a load other than the sail load? Have you looked with binoculars at the top of the mast when it is binding up to see if the halyard is in the shive and the alignment of the halyard is correct going in and out of the the shive?

Sorry for all the questions, This is something that just requires some analysis on your part. It could also be the shive at the top of the mast is the culprit when under the load of 10 foot of sail it stops rotating smoothly. Do you have another halyard up there (spinnaker) to try hoisting the sail with.