That is EXACTLY what is happening. When you check "Attempt to repair minor problems" under the Integrity tab, the maint plan job will try to change the database in single-user mode before the DBCC command. If it cannot change to single-user mode (because another user/process is logged in), the job fails. However, if it can change to single-user, but someone else logs in before the DBCC command starts, then the job fails, and the database does not change back from single-user mode.

Recommendation: Do not select "Attempt to repair minor problems" unless you are absolutely sure no one else, and no other process, will use the database at that time. Perhaps middle of the nite, before backups might be good. If database is used 24x7, do not use that feature at all.

Mark