4 SoFree give 100% chance, as Marsu said. Probably your Wyrms were hit with an area spell, like Firestorm, since it doesn't "target", SoFree can't make Wyrms evade it.
I always run Chaos Wyrms with 6 SoFree, just for extra draw chance. Things get really nasty when you can't get all the Shards out and your Wyrm gets Congealed.