Same here. It has come to a point where I believe Konami designed the system to reduce the drop rate of Princess all the way down to 0,001% if you are using Marik or Ra.....
it's all an RNG, first day didn't get nothing i play with Marik and Ra, Skill: Tribute sacrifice something like that, and guess, i just lost to her level40 and she droped an UR, I hope you will get more of her soon just keep trying, my All the luck be with you guys :)