It really does seem like the drop rate gets turned up and turned down sometimes by Innogames--but who knows--variance is variance. You will go a week or two sometimes without getting a bp drop, it will happen, and is unaviodable. You will also score 4 a day sometimes, though rarely.
If you p/m 80 guildies, and almost 140 friends, plus plunder neighbors, Nobull's assertion repeats my experience--you'll get 2-3 a day on average. That's getting close to 300 actions per day, so it seems roughly under 1% of the time you will get a bp drop, thats it. They're not supposed to drop from the sky.