Probably because it's too tight against the grip, stopping it from rotating back?
You might need to undo the screws in the throttle and slide the throttle closer inwards, up the bar. Once you've got a little bit of bar at the end, try your new bar ends again.
Probably just needs a good fiddle!
I'd rather die peacefully in my sleep like my Grandfather, than screaming in terror like his passengers.- Jim Harkins