Inattentiveness is an offense—one of the offenses to the Holy Name. It is called pramada. Do you know what pramada means? It means madness. Bhaktivinode Thakura says that even if one avoids all the other offenses, but is still inattentive, then he cannot achieve any real success in chanting.
MKV, Vol 2- Pg 14 (Q/A Meeting With Disciples, Divnomorsk—6/10/01