Why is my 301'd domain still ranking in Google?

I had an older domain that was spammed, so I copied the site and uploaded it onto a brand new domain. Then I used a 301 redirect to forward the old, spammed, domain to the new one. But its been 3 months and the old domain is still the one ranking in google (page 3-5), while the new domain is nowhere to be found. When you click the link it still forwards to the new domain so I know the 301 redirect is working correctly. But why does the old domain still come up in google?