Can't sync Safari bookmarks on all devices

I can’t get Safari bookmarks to sync correctly on all my Apple devices. I have an iPad 5th generation and iPhone 6 both running iOS 12.4.1 plus a mid 2011 iMac and a mid 2009 Macbook Pro both running El Capitan 10.11.6. The strange thing is that while Safari history syncs correctly on all four devices the iPad and iPhone will only sync their bookmarks between one another and not show the bookmarks on the iMac or Macbook. And the iMac and Macbook don’t sync with each other or show the bookmarks on the iOS devices. All devices are on the same iCloud account and all devices are set to sync Safari. I have carefully gone through the troubleshooting tips at https://support.apple.com/en-ca/HT203519#iOS and have gotten nowhere. Any suggestions would be appreciated! Thanks.