Well a couple things here:
1. You may want to get your own merchant account at this point. If you are expecting 15-20 sales daily from two smaller sites then you have enough volume. Maybe it's time you took control of your own billing.
2. There's been an overall drop in conversion ratios since about a year. I hear this from many webmasters (affiliates) and Program Owners. Nothing had changed on my sites and the traffic is increasing so what could be the reason?
Mobile.
Devices are changing people's buying habits. Many check out a site on a tablet or cell phone then signup at home on a desktop. But those same mobile users buy this app and that Membership and this Upgrade and so on, tap tap tap. So many go over their limits without realizing it or do so much micro activity that their banks slow down the approvals.
I found by changing my CCBill signup forms to my own custom responsive forms sales have returned to near normal levels. I would talk to CCBill about turning your forms responsive and seeing how your sites do on mobile. Maybe offer a $1 trial on mobile only that converts to a full Membership.
