Email Subscriber list not growing anymore

[mrb.frequency]

I have a banner installed in my blog for those who want subscribe in my blog. The banner shows only on those pages those who gets organic traffic from the search engines. It was working fine I was getting atleast 5 subscribers per day. But since last week I haven't get any new subscribers. I am getting traffic to those pages but no one is signing up. I had checked the banner is working just fine. What should I do??