Ive gotten all the event girls. I know it seems like they will never drop, but every hour i use all my bp. Most of the time they drop just before i go sleep. Some were close calls, dropping on the last day. I alternate between the 2 trolls, and that works for me.

I think you can increase the odds. You need to have a specific booster that raises luck. I have a Chlorella. A Epic item from a quest at the start of the game. Never used it, but it does raise luck. Like 1200 points for 3 hours. You cannot buy it, but you can sell it for like 2k.