For a brand new backup set doing its first backup, the first backup (online or to USB drive using Copy to Disk) is always Full regardless what Backup Type you pick - Full, Incremental, Differential or Bit Backup. It backs up everything selected, regardless of whether it is marked as "backed up already" (archive bit OFF) or not, if you have selected Archive Bit as your file selection method. Then it will automatically switch to the selected backup method for subsequent backups.
For example, if you start out a new backup set as Incremental and Archive Bit, it will back up everything you have selected and then switch to Incremental mode, which backs up just the files that change or are added between backups.
So, if you select additional folders or files AFTER the first full backup, the software will handle them according to the Method you selected. If you selected Incremental, it will back up only files that have the archive bit set.
If you want it to back up all files that are selected AFTER the first full backup, like I think you are asking, then set your File Selection to anything other than Archive Bit. Set it to Modified Date/Time or Fast Pick. You will find this in the hidden Properties section of the Scheduler screen on the Client, and you have to enter the Administrator's password to see that section. See the Server's documentation for info on how to access that.
If this is how you want it to act by default, we suggest you set this as the default in your Customize Client Installer section of the RBS Manager, then rebuild your client installers.