Sorry, don't know, why your LDs are fading to blackness. Mantras can be very powerful - try something like "My lds are long" "I wake up only when I want to", or anything like that and you have to believe in power of your mind. It will work.

WBTB - have you tried moving your alarm to the other side of your room? Have your dream journal or piece of paper with your goals for a lucid handy, so you see it when you wake up. That should get you excited about it. You can draw something cool about your lucid goal, a dreamscape thats nice and brigth and write on it 2 min long, or something like that. Make that picture special.

You could check out this collection of methods for DILDs to see, if there is anything you could add to your practice. Hope this helps