A few attempts (up to 700 maybe) aren't enough to statistically determine the rate at which an event is occuring. Along those lines, a few attempts won't necessarily let you see an increase in soulcrafting rates.
Also, may I suggest a way to reduce the amount of unsatisfied players and make it feel more like an event ? Try calculating the needed amount of souls, gold and insurance scrolls needed in order to increase the grade of the item in average*, and let people use half of said amount at once in order to have a 100% chance to raise its grade. It would be absurdly unlike any other MMORPG, and gathering this huge an amount of items may look difficult, but it would let players use an exact amount of items instead of rolling a few dices and hope it works.
*Think of it this way : 10g + 1 IS + 500 souls with a 5% success rate (average number of attempts needed : 20) => 200g + 20 IS + 10.000 souls divided by 2, so in the end 100g + 10 IS + 5.000 souls.
You could apply any soulcrafting bonus to the total amount of items, too.