I mined with suprnova a lot, and rewards are, or, were very good. Although some minor server problems due to ddos attacks on it, everything seamed to be going fine.
You said your rewards should be 0.015, but you have to keep in mind, rewards are more or less luck, and luck is a lot more noticeble on smaller pools, like suprnova. But it all evens out in the end, today smaller rewards, next day, bigger… usually how it goes. You can not judge a pool by just mining 24 hours on it, you would need a sample size of at least a week, more or less.
You could see pool stats here: https://poolstats.org/ZEC