I use an enchanted bow that traps souls but have run out of arrows. When I use new arrows the soul trap doesn't work.

How do I get some more enchanted arrows -- they do not appear as avaiable when I use the Arcane Enchanter.

This is very odd. Arrows cannot be enchanted, the soul trap you have on your bow should work with any arrows you use.

Were you trying to soultrap summoned creatures or corpses?

I dont see anything on the wiki about this. This may sound like a silly question, but are you sure the bow has enough of a charge on it?

Well what happened was I killed a bandit with an arrow and got his soul ( lots of wavy lines on screen told me I was successful). Then I one-shot killed someone else with a new arrow and nothing happened, using the same bow.

Yes it,s still about two thirds full.

What about gems. NPC souls can only be captured with black gems.

Yeah, you probably dont have the right kind of soul gem.