Why are web site sign-in passwords not syncing from MBP to iPhone of iPad?

I have looked through the Safari user guide and checked that I am signed in with the same Apple ID (fairly obvious, that one) and that two-factor authentication is on as well as syncing to iCloud Passwords and Keychain on all three devices. (Keychain appears to have been replaced by Passwords: I cannot find it on the MBP.)

Most data appears to sync, but I need to sign into some web sites, and not all of their passwords are available on the iPhone or iPad. While the synchronisation between Apple devices is generally pretty good, and extremely useful, this is becoming a nuisance. If anyone could shed light on this, I'd be grateful.

pqb