For he that is dead is freed from sin.
Note 9 at Ro 6:7: There is a difference between being "freed" and being "free." In the 1860s, President Abraham Lincoln issued the Emancipation Proclamation that "freed" the American slaves. Many slaves, however, continued to serve their masters in slavery because the truth was hidden from them or in some cases, the slaves were afraid that they couldn't make it on their own.
Likewise, Christians have been "freed" from sin, but that doesn't automatically mean all Christians experience that freedom. Through ignorance and deception, Satan continues to maintain mastery over those who have not yet realized their death and resurrection with Christ.