When you set a differential or incremental backup, and tell the software to delete backups older than 15 days, on the 15th day, the software will delete everything and start fresh by doing a new full backup, then incremental or differentials after that, therefore on day 16, you will only have 1 day of backups, instead of the 15 you need. WE ARE WORKING AT A NEW WAY AROUND THIS SO THAT IT WILL ONLY DELETE ONE DAY AT A TIME.
In the meantime, the good news is that there is a way around that. If you want to keep 15 days at all times, you will need to schedule 2 differential or incremental backups that expire EVERY 30 DAYS.
You will need to run the first one first, then 15 days later, run the second one both set to delete any backups older than 30 DAYS.
In that setup, backup two will start on day 15 for 30 days. 15 days later, backup one will delete itself and start over, but since you started backup 2 on day 15, you will have 2 weeks worth of backups on backup 2.
On day 45, backup 2 will delete and restart, but since backup one deleted and restarted on day 30, you will have 15 days worth of backups on backup 1 (see diagram below)

The only bad news is that you will, in essence, have 2 full backups at all times, therefore, you will need twice the amount of space, but this will work great.
As I said, we are currently working for a way around that, so that you will not need twice the amount of space. We will let you know as soon as we do so.