---Log in to Plus and stay in Plus. If you go back to SA (either deliberately or by chance [it happens to me if I click on a link in the newsletter, or sometimes when I search]) you will need to clear your SA cookies.
After you log out of SA, clear cookies, log in to SP, your sweeps pop should work smoothly. If not, let us know.
In the past, some people reported similar problems using Chrome. Are you using Firefox or Chrome? If you are using Chrome, you may want to try Firefox and that may solve the sweeps-not-popping problem. Here is a link to a thread where Plus members discuss this, and their solutions:
Hope you find it helpful.