We accumulate (store up) pending vote value for each member until it's high enough to support a minimum upvote of $0.012. 

This is explained in our FAQ and in more detail here: 
https://steempeak.com/steembasicincome/@steembasicincome/value-balances-and-upvote-delivery

At your subscription level, and with STEEM prices so low right now, it takes time for your balance to build up enough for each upvote. That's why we recommend at least 40+ unit membership level (and offer some additional perks in our Discord to encourage people to reach that level).