Hmmm...that could be an issue with the bar end weight squeezing the rubber of the throttle. You'll need to loosen the bar end weight with an allen (hex) wrench, pull it out, see if that fixes the stickiness. While it's off, might want to squirt something in there...like graphite (?) to help. Then, put the bar end weight back on, but not as close to the throttle.
Sometimes the brammo guys drop in with advice,.but they are out of the country right now.