Keeper.sh now rechecks each connected account for new calendars on a schedule. Before this release, the list of calendars on an account was read once, when you connected it, so a calendar created later only appeared after you disconnected the account and set it up again.
Calendars that appear later are picked up on their own, and calendars that were deleted stop being offered. Your existing connections do not change: the recheck keeps the list of calendars you could sync current, not the syncing itself.
Each account also has a refresh action that rechecks it immediately and tells you what it found. Use it when someone shares a calendar with you and you want it in Keeper.sh straight away.
Linked accounts and calendars explains what you can do with each calendar on an account.