Some sites use an autocomplete=off option, which means that some (but not all!) password managers will not attempt to remember the username and password. Dropbox uses this, for example.
Safari’s password saving feature will honour this, meaning that if you visit Dropbox you have to enter your password each time. It also means that you can’t automatically add that site’s username and password details to iCloud Keychain. Essentially this means the site or service can’t be remembered by iCloud, so you can’t share the details across devices.
However, a really simple extension from Lap Cat Software (a.k.a. Jeff Johnson) tells Safari to ignore the autocomplete attribute, with the result that you can now autocomplete on sites like Dropbox, and also therefore add these login details automatically to iCloud Keychain so that they’ll sync with your iPhone and iPad.
Here it is. Download it, then click to install it. It represents a security risk because we’re trusting Jeff hasn’t inserted any malicious code. But others have been using this for quite a while and reported no issues.