I always thought something like this would work:

http://dota2.gamepedia.com/Pseudo-random_distribution

Yes, I am linking DotA2 mechanics, but the "RNG" in DotA2 ensures that while the probability over time is similar to what is listed, you're guaranteed eventually to get it within -set number- of times, depending on the initial %. One drawback of such a system would be a lack of back to back drops for those lucky peeps, but then again, we're complaining about LACK of drops, not back to back drops.