Confused about update frequency settings

[wedens]

I've read multiple issues/discussions on the subject, but I still can't get a combination of settings that does what I need. I simply want to update all feeds every 15 min.

My current options are:

POLLING_FREQUENCY=5
SCHEDULER_ROUND_ROBIN_MIN_INTERVAL=15

Some feeds are updated every 15min (as expected), but other feeds are updated only once an hour.

What settings do I need for every feed to update every 15min?