I have been working on Photoshop CS6 for a while now and this is some basic text I did with overlay and a few other settings:
But now only the normal mode works, but when I put on overlay it just vanishes:
Overlay isn't doing anything for you, because your base color is black.
Overlay needs some color information, other than solid black/white, for its effects to work.
Overlay affects the layer beneath it by multiplying and screening at the same time. This means that areas less than 50% gray will get darkened and areas above 50% will be lightened.
Your problem is that black can't get any darker and white can't get any lighter.
I don't know the science behind it and maybe someone else can chime in with that part of this, but there are certain colors, when fully saturated, that will not be affected by
Overlay. See below (poor quality thanks to Zamzar).