Your interval of 24 hours means that an incremental file backup will not begin before 24 hours have passed since the last successful file backup of either type. The interval for full file backups is compared only to the time since the last successful full file backup.
Once that interval has passed, the backup can only be scheduled to begin during the next general backup window, or incremental file backup window if you have chosen to have different settings for the four types of backups.
The general backup window you listed means it will be able to attempt an incremental backup between 9 AM and 10 AM at least 24 hours after the previous backup finished. If the previous backup finished after 10 AM, the following backup window will close before the expiration of the 24 hour interval, so it can be 48 hours before the next backup may be scheduled. (That’s why I prefer to set a 23 hour interval so missed days happen less frequently.)
Another reason you may miss a backup window is if the client is not running during the times you specified. There is an Alert setting to send an email if a client misses several backup opportunities in a row, so you can take action to fix the reason the client is not ready. There are also notifications available for failed backups so you can fix those problems as well.
More rarely, a Client may miss a backup window because the Server is busy doing backups for other Clients and does not have a backup slot available to start an additional backup. The Max number of simultaneous backups setting controls how many backups may be attempted at once. The default value is 10, but you may want to reduce that number if your server or network resources are being strained by running 10 backups at once. There are several other settings to help ensure UrBackup does not cause problems for other tasks or users. Check the Administration Manual for details.