The amount the curators get is at best 25% and how it's distributed among the curators depends on their Steem Power as well as when they cast their upvote. Being among the first to upvote a post will yield a higher reward than when voting after a lot of people have already done so. Additionally upvoting a post within the first 30 minutes of the posts existence will transfer some of the curation reward back to the creator. This amount decreases linearly from 100% going to the author when upvoting right as the post was created down to 0% at 30 minutes.

If you didn't receive a redeemable amount of Steem Power from your curation that's because your part of the share was less than 0.001 Steem Power and was rounded down to 0. However depending on the factors mentioned above the 15 Steem Power you start with are enough to get rewarded for your curation. Also make sure to keep your current voting power in mind, the more you vote the lower it gets and the lower your impact and reward will be, your voting power automatically regenerates after a while. You can find out what your voting power is at any time here: https://steemnow.com/

Hope I could help!