Rollover will only work when you buy a data plan with the same validity. It won’t work if you buy 3 day plan and later 7 day plan.
For example: John buys 3 Day 500MB plan 3 days ago at 2:00pm. He only used 400MB of the bundle and plan expires. In order for him to retain the unused 100MB, he must buy any 3-day data plan within 24hrs from the time of expiry.