It just takes practice. You should be thankful for the experiences you have had, they are very promising. WILDing is just like meditation, you shouldn't have a goal or be disappointed if you don't reach it. Every session you do, whether resulting in a lucid dream or not, helps you to get better at it. I am only sometimes successful at WILDing even though it is my favorite technique. Most of the time I believe it helps me to DILD anyway. Only about 1/3 of my lucids are from WILD.

Also, I find if you combine WILD with VILD (visualization induced...) is very successful. I visualize many things like all of my meridians glowing and feeling warm and I visualize every shade of every color. I visualize something repetitive like slowly climbing a spiral staircase infinitely upwards. This works well to keep the mind bored enough to fall asleep and engaged enough to stay conscious, or the right balance in between. That balance is hard to find.

Also, I think since you only spent 15 mintues for the session you didn't give yourself enough time. It pays to be patient. I always think along these lines: Even if I don't fall asleep tonight I will stay aware and conscious. If your body is the utmost relaxed it WILL fall asleep. And you will be rested in the morning. I never actually stayed awake all night attempting to WILD but it helps to be willing to just to develop patience. 15 minutes probably isn't even enough time to fall asleep normally especially at 2 oclock (pm?). If at 2 am I would suggest going to bed earlier and not eating right before you go to bed. Your energy is busy digesting food thus bringing your chi (life energy) to your physical body rather than your dream body.

Just keep experiementing and practicing. Even unsuccessful WILD attempts should be fun. It took me every night for a few months before I was successful at it, and now I have the knack for it.