I was doing the same thing, trying different stretches etc to acheive the full lotus position and I was getting there, but slowly. No matter what I did I just couldn't quite get it. One day I was about half an hour into a meditation session (in half lotus) and I simply reached down and grabbed the other leg and it went easily into the full lotus position.
Being in this very relaxed state helped me to acheive the full lotus. I was already very flexible however, having trained in several styles of martial arts for the previous 7 years.
Remember to always warm up before trying any stretches to prevent injury but also to gain the most from them.
Best wishes
