You have only 1 unit subscription level. With the low STEEM price, 1 unit is only enough to support an upvote every few weeks.
4 units would get you up to once per week and 28 units would be enough for a daily upvote. We usually recommend a subscription of at least 40 units.
To clarify, I gain additional subscription units by sponsoring more members into the program, correct?
That is correct. Each time you sponsor somebody for SBI, you also get additional units added to your subscription.