Its been mentioned at least twice that the vang needs to be loose but the OP already mentioned the vang. No one mentioned the downhaul but I already assumed the OP has checked every piece of running
rigging. If the halyard can be raised and lowered easily with a tagline (without the sail rigged) with increasing tension placed on it then he knows it is the sail track or slides or slugs or boltrope.
If you have external track then just a little jog at a joint can mess up raising and dousing.
kind regards,