Source merits are a constant flow, and not a monthly update. They are replaced 30 days after being awarded. If they aren't awarded instantly, they aren't lost, it just means a slight delay in their replacement.

If they complain about amounts, tell them to complain to me. It's best if sources try to exhaust their source allocations, even if it means giving posts higher amounts than is typical. If you have 150 source merit and you only see 3 merit-worthy posts in a month, then I'd rather you over-give each of them 50 merit than let the merit expire. That way there are more people capable of sending merit, and the "merit economy" is less top-down.