Have old bar end shifters which don't seem to fit in the new handlebars
I think your picture shows that the expander wedges are riding up on the ridged part of the shifter pod, which keeps them from being able to get small like they're supposed to, hence not fitting the bar they just came out of. You need to twist them around so they're not riding up. Also at the very least grease inside the bar, but ideally grease the threads and every surface of the expander wedges.