You should check that the Dumpster folder is enabled at the account. You can change settings at account. In COS can be Enabled, but you can change it directly to choosed account. So check if your account has really Dumpster folder enabled.
You haven't to ask if you should restart zimbra server. If you are know that you have done what you should, just do it and you will see
What you should do is logout and login by the webmail user.
It is working for me in 7.1.1. In 7.1.0 were some bugs with Dumpster. I think that dumpster working only in Mail in 7.1.0. In 7.1.1 with patches Dumpster working fine for me.