I'm just learning this stuff too, but I've figured out what the 'disappearing runway' on ILS approaches is about (i.e. when the runway is not shown on the legs page of the CDU).

If you've selected an ILS approach, the plane knows that it's going to have to land on the runway and it will do it by itself, - you just need to control the speed (on the autopilot panel) and the flaps (gradually, by increments). If you're too late pushing the App button on the autopilot panel, it won't pick up the glideslope/localiser and will probably sail over the runway without landing. I recommend pushing App before turning onto the final approach heading.

Then you need to check that you don't have a crazy vertical speed (of descent) happening on the autopilot (if you do, set it to 0fpm). You might also have to check that no altitude is constricting the flare in the autopilot altitude window. It's probably a good idea to zero both of these before the final approach so they can't possibly iterfere and override the ILS.

This is the current state of my understanding, and it's working for me, - hope it does for you too.