For the first one, the idiom is actually "it escapes me" and it's meant to be used when you can't remember something in the present time. The second idiom is: "It slipped my mind" and it's also referred to something that you forgot about or can't remember. "It doesn't come now" is just not an idiom or a phrase that people use. People usually say "it doesn't come or spring to mind" which has the same meaning as the ones before it, but people usually use it in affirmative ways like "come/spring to mind" which means you recollect or remember something in particular. So, none of these idioms are things you'd normally say when you don't have anything to say or you don't know how to respond to someone or something. For that, we usually use "I'm speechless" instead.