I ran into this problem--
the bar end lever literally has almost exactly the same inside
diameter as the soma branded bar-end levers outside diameter.

I'm not saying
you should do this but I gently encouraged my lever in with a
framing hammer and a piece of wood to shield the lever from
the metal blow--tapped it in. I was concerned that the
lever would crack the tubes on the bullhorns. . .but it didn't.

Check to make sure the internal diameter on your bars
fits the "19.6 to 20.6.mm" internal diameter the
levers are spec'd for (from the Soma website).

I was annoyed--I had reverse-wrapped the bars and wanted to tuck
the tape into the bar end and plug it with the lever--no way with that clearance--
had to tape it.

edit: make sure the bolt that tightens the internal wedge is loosened--
the lever is designed to go in and then wedge against itself as you tighten it
up--if you jam it in there while the wedge is snug up against the lever to begin
with you will defeat the mechanism's function.

anyone feel free to chime in--this worked for me but I ain't no mechanic.

Good luck.