Did you check the Dungeon of the Succubus Event page? No reward is distributed if you have not at least completed that step.
Otherwise, I'm not completely familiar with the rules about changing guilds during events, but that you lose eligibility for changing guilds too close to the end of the event seems like the most likely possibility.
How the Hard did you read that post? O_O;;
Well, these games could be somewhat unstable with their data, due to not having several servers backing them up (a GM is free to prove me wrong), so the only real "guaranteed" way to getting counted for guild rewards is stay consistent with one guild. Heck, there might even be a bug that if you leave a guild, you're pretty much screwed for the prizes in the first place.
I suggest contacting support with this issue, except with better grammar.