How do I sync Safari's Reading List between multiple Macs and an iPhone?
Sometimes this can be caused by temporary issues with your account or the connection to the iCloud servers. Waiting it out isn't a bad strategy if you don't want to go for official support or when restarting the affected devices is not effective at clearing up the issue.

For ongoing issues, you might reach out to iCloud support to ensure it's not an issue with your setup or account.