I've seen a couple of the cutaway animations fail to trap souls when they go off, but for the most part, they work reliably. As others have pointed out, a longer duration soul trap helps. I have my Fiery Soul Trap set to 4 seconds, works great.
Is this based on the enchant power or something? Just made character haven't started enchanting yet.
Yep. Do the following:
1) get the Black Star
2) Kill a bandit
3) Disenchant your current soul-trapping weapon, and manually enchant a new weapon with a 3-4 second charge.