thats a tough one without being able to see it but check that the cable is in the groove in the twist grip and that all the parts are where they should be inside there. loosen the screws a turn or two and see if it will turn you may just have the plastic washers in the wrong position, good luck frank
is the slide in the carb correctly? theres a groove on one side that goes opposite of the screw and a notch thats goes nearest the screw. with the filter off have someone slowly and gently twist the throttle with you looking in the carb inlet to see if the slide goes up and down,or you can do it yourself if you have long arms like me.
plus you have to make sure your cable not binding dont tie it tight to the bike frame.