If you have multiple devices associated with your iCloud account, make sure that each device is logged into the account and that all devices are using the same Apple ID.If you're using a shared iCloud account, make sure that everyone in your household has authorized their devices to use the account.If you're not using iCloud, make sure that your devices are connected to the same network and that your Mac and iPhone are on the same Wi-Fi network.If you've tried all of these tips and your reminders still aren't syncing, please reach out to our support team for help.