This is similar to Paul in 1 Cor 3. However it seems that the writer of Hebrews is concerned that they need to keep hearing about repentance and forgiveness. I sometimes feel the same way about myself. I keep needing to repent as God reveals various sins. On further thinking though I'm not sure that's what this passage is on about. A mature Christian will not be sin free, but they will know what to do when they fall into sinfulness. So for example if I was immature and sinning in a certain area, I would not probably need to be told that I need to repent rather than taking the initiative myself.

Hebrews 6:1