How do I force Safari to ask me to save all passwords? I'm using 10.10.3. iCloud keychain is enabled. 

Anyway, I find that Safari will ask to save passwords for some sites but not for others.

examples of sites where I get prompted: eBay, amazon, some local discussion forum, stackexchange

examples of sites where I don't get prompted: macrumors, xdc-developers and many others

I've checked the settings and "Auto-fill username and passwords" is enabled i the password tab. I've removed all passwords but it's still happening.

Any ideas on how to fix it?
This is, technically, not an answer but might be a reason why some sites do not save passwords.

They (those web sites/pages) have code in them to tell the browser to not save the password. 

I used to have a JavaScript bookmarklet that would (with varying levels of success) remove the code that said "don't save the passwords" but when I moved over to 1Password I stopped needing that.

Possibly a password manager might do that for you (there are free ones, I believe) or a bit of google-fu might find you that bit of JavaScript that worked some of the time in telling sites to stop telling your browser to not save the password.