Safari's Autofill concern

I have AutoFill enabled and it has captured many of the User Name - Password combos I have entered over time. However, there are several sites I visit regularly where the AutoFill feature does not offer to capture the log-in info, e.g., MacRumors.

Is there a way to "prod" AutoFill to capture these info pieces on a case by case basis? Are there some sites that do not permit AutoFill to work as intended?

Thanks for your thoughts!