I've been unsuccessful in my recent attempts to create a weapon enchanted with the soul trap spell, and have no idea why.

My character knows the spell.
The weapon is enchantable (Ebony Mace).
I have a gem filled with a soul (common / common).
All this is taking place at the Arcane University, using the proper item (the Enchater station or whatever it's called).

Has anyone been successful in enchanting a weapon with soul trap?

yes, i was able to enchant adeadric bow with soul trap, however i have couple of spell and enchanting mods active so the station i used was not in the mages guild this could be the reason.
try using a bigger gem i used grand/grand

What level is your Mysticism? If you're a Novice at it, I don't think you'd be able to cast Soul Trap, and that might affect your ability to enchant weapons to cast it.

You need to do the spell at least once it happened with my water walking boots and had to get up to apprentice alteration

all you need is a basic soul trap spell that you can cast if you cant cast the spell you cant enchant the weopon, the first soul trap spell if i recall needs a mysticism of 25 and is bought in i think chorrol or cheydinhal.

I would definitely use a grand level soul stone for any enchantment. Save the smaller ones for recharges. You get much better enchantments with a grand soul.