Check your account settings on both devices and see if you have Notes enabled for all the same accounts.
Whether you have an Accounts button at the top left depends on whether you have Notes turned on for more than one account under Settings > Mail, Contacts, Calendars. If you only have one account enabled for note syncing on that device you will not get the option to switch between accounts, or choose All Notes.
iCloud.com only shows itself, of course.
If you have Notes enabled on another account you'd need to go to that account's website to see them. In most cases the Notes are found in a special folder in that accounts email; named Notes, of course. You should not edit this folder. Any direct editing of items in the Notes email folder will break that note. The folder exists for syncing (between devices) only; though it's fairly safe to copy (not move) notes from this folder.
Be sure to go to Settings > Mail, Contacts, Calendars; the Notes section, and select a default account. This is the account that Notes will end up in if you don't specify/select a specific account before creating them. Siri will always create notes in the default account. If you create a note while looking at All Notes, they will be created in the default account.
Like the Accounts button, you will only see an option for a default account if you have Notes enabled for more than one account.
