Merit is for quality posts and if there are not enough quality posts, source merit can be spent less than before.
The solution is to just give more Merit for each good post.
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.
I can't say I completely agree with theymos here, but I get the reasoning behind it. If all Merit sources would do this, the amount of Merit created each month would be equivalent to the total of all Merit sources.