I have included a drawing. In it I have created a sort of slanted block. I want to trim off the top of the block. I've tried several things to trim this block with no success. Finally, I made the trimming planar surface through the block, but it still won't trim off the top of the block. These are fairly simple objects. I just can't see any problems that would cause this trim not to work. Can someone spot whatever it is that I'm missing? It's frustrating to get hung up on such a simple operation.

Thanks,
Mark