Maintenance Plan for all Online databases doesn't execute for all databases Using SQL Server 2008 R2, I have a maintenance plan that has a basic Backup All Databases (ignore offline) task, and clean up after 7 days. 

However, I note that sometimes I lack backups for (seemingly) random databases. 

Even though the script says all databases, bar offline, is there some parameter set where databases won't be backed up? E.g. No changes to database since last backup, or some other conditions?

The particular database in question at this time has been updated in the last 24 hours, but hasn't been backed up in the past week. 

edit: the recovery model is set to Simple, which is the same for most other databases on the server, so it's probably not that. 
I've tried to look for other documentation, but I can't seem to find any technical details, only high level business/sales white papers.
In your Backup task, is your Backup Type set to Full?  If it is set to Differential then a backup may not be taken of every database.