Why won't Safari ask me to autosave my passwords for some websites?

Under passwords in your autofill pane, make sure the site in question isn’t set to never remember. Several mac forums suggested that certain sites you can visit will turn off the autofill feature, well… automatically.

Also this (and I quote):

I have found a solution to this problem on my computer that has so far worked 100% of the time. First, open keychain access, and if the keychain viewer doesn’t open up right away then go to the top of the screen and click on “window” and scroll down and click on “keychain viewer”. Once keychain viewer window is open make sure in the upper left hand corner under “keychains” that “login” is highlighted. Then, under the word “category” make sure that either “all” or “passwords” is highlighted. Usually I just choose “passwords”. Then highlight the name of the website that you want to be able to your passwords in. Next, doubleclick on this website name. When you have opened up the website the default at the top is usually “Attributes”. Next, check the box that says “show password”. At this point you will probably be asked to type in your keychain login password. Do this and click “allow”. Next click “Access Control”. Then choose “Allow all applications to access this item”. Next click “save” to save the changes. You will probably be asked again at this point to enter your keychain login password. Do this and click on “allow”. After doing these steps passwords will usually be saved at the website address that has been chosen. If an error message occurs at any step along the way, a workaround is to quit Keychain access and start the process over. If an error is again encountered, the best thing to do is to highlight the website name in the keychain viewer and then go up above to “edit” and delete the websight name. Then you can go back and manually reenter the website name. This is done by clicking on the ”+” symbol at the bottom of the keychain page. However, before deleting the website name, make sure you open it up and copy all of the relevant information from the page so you can reenter it. Another thing that needs to be done is to go to Safari preferences and then click on “autofill” and make sure “user names and passwords” are checked. I made sure all three of the autofill web form boxes were checked.

that helps thanks ;)