Define the payment rate. That way, you get people who will give you what you want.

20k per month for 90 articles? I think that's way too small. I charge clients N5 per word. Even if I go N3, the 20k isn't even up to half of the money for my charge. I think you should review your payment rate or review the number of articles per day as well as the number of words. Writers are not sewing machines.