[Radek]

Sometimes, the spell does not work because you haven't touched the victim - you were too far from it. The remedy is creating a ranged Soul Trap. Ranged Soul Trap is much more confortable than the normal Soul Trap and it isn't much more expensive than the normal Soul Trap. You get the following advantages:
1. You cannot be too far from your victim. The spell is ranged.
2. You can cast the spell even if you are close to your victim. Ranged spells work at the "touching" distance, too.
3. You can cast the spell on your victim while it is far from you and then you can catch up your sword and fight. This system is much more safe than trying to cast the spell in the middle of the fight.
You can also increase the duration of the spell, for example, to 90 seconds. The spell will be cheap still.

Note: Another possible reason for the failure of your Soul Trap is magic resistance of your victim. If the victim has resisted the spell then you get no soul.