Re: Login check box
Keeping you logged in is done with a cookie, a small piece of data stored on your computer. Like everything they can be used for nastiness for everything from viruses to targeted advertising.
Because of the risks some browsers or security software will not allow cookies to be stored, or delete them. I can't begin to guess at what is affecting them for you, but hopefully that will give you an idea of what to look for, to disable, enable or allow.