Be sure you use the same selection list for both your full and incremental backups. Also be sure that both the full and incremental backups are set as 'Using modified time'. When you run a full backup, a reference 'time' is saved. When the modified time incremental is then run, it looks for, and only backs up files with a later modified time than that saved from the full backup job... it works. Be aware that if you install new software or copy files onto the server, their modified times may be older than the last backup and will be missed during the incremental. A full backup should be run in this instance.