It's normal.
There are so many Junior member have only single merit, that help them to get that rank.
Whats going on with your merit? 1 per 910 post?
I have something to contribute to your project and the campaign. I am not on computer now, in bed, instead, so I will write down shortly.
1) Payment rates: it is low but the low payment rate is not matter to decide your campaign will get supporters or not. It is just one of factors to attract them.
Two factors attract supporters are:
Payment rates: that likely can not be raised more (you already raised it higher than original ones), because your funds for it maybe small, I guess.
However, you should not worry about that. Even some $ means lots of things for people around the world, especially who lives in developing, low income nations. It means a lot for them, as well as your small site. Such low funds to pay for supporters maybe come from lots of efforts of Microlancer team to do promotions.
Post required to get paid: with such low payment date, 20 post per week is highly requirement, I think.
So, you should reduce post requirements a bit. Addiitionally, more posts don't equal to more attention.
If your participants mainly posting shit posts to get quota, it might lead to failures for your campaign.
Readers might feel annoying with your site, that promoted by shit posters.
2) Post quality: you should focus on this. Such as post length, acceptable boards/ child boards, and don't accept off-context, off-topic posts. This one should state in the rule of campaign as detailed and strictly as possible.
From the OP:
Only productive posts will be counted (helping people, opinions or thoughts no 1 word bumps, or 1 sentence post, no filibusters)
Rule on post quality is not good, and should be adjusted.
3) Manual checking post quota and post quality: participants who don't satisfy post quota, and / or post quality will not get paid. It will help maintain good post quality in general for your campaign, that in turn result in better effects. It will help the team to save more money by reject to pay for shot posters and their shitshows.
Nearly $80 in total per week, for 6 weeks, it cost around $480, that is not a small fund from small site.
Let's do it right for your project.
I don't read rules and the OP carefully, can not quote to discuss because I am in bed, sorry for any inconveniences.
To sum up, I think ten (or 15) posts per week is fine, but focus on post quality, and don't pay for low quality posts as well as burst posting posts.
Simply do what you think is right and good for your site, your fund, your campaign.
Should focus on quality rather than quantity.