Work through these in order. They are arranged by how often each one turns out to be the cause, not by how likely they are to sound impressive.
1. Does the entry have a web address on it?
AutoFill matches a saved entry to the page in front of you using the address stored on that entry. An entry with no address will never be suggested anywhere, no matter how correct the username and password are. Entries typed in by hand are the usual culprits, because there is nothing to fill the address in automatically.
Open the entry, add the address of the sign in page, and try again.
2. Is AutoFill on for this device?
The AutoFill provider setting is a system setting and it is per device. Turning it on for your iPhone does not turn it on for your Mac, even with sync on, because it is not part of the vault. A vault that syncs perfectly and still never fills on the new iPad is almost always this.
3. Is the vault locked?
A locked vault has nothing to offer. The vault locks itself after a period of inactivity, so this shows up as AutoFill working fine in the morning and not at lunchtime. Open the app, unlock, and go back to the sign in page.
4. Does the address actually match?
Plenty of large services sign you in on a different domain from the one you browse. If the entry says one address and the sign in page is on another, there is no match to offer. Adding the sign in domain to the entry fixes it.