[Koha] Fines always zero

Hello, I am a relative newcomer to Koha. My system (Koha 17.05 on Debian) is not calculating fines (they are always zero). I have checked: 1. There is a non-zero fine set in my default circulation rule (the only rule at the moment) 2. The cron jobs seem to be running ... e.g. I see a backup file created at 06.25 each morning. 3. I have also tried running /etc/cron.daily/koha-common manually but the fines are still zero. Any suggestions on the likely cause, or how to investigate further? Thanks! Chris Brown