Hi Lordfox, can you please post the 3DM model file with the surface you're having trouble trimming? It's difficult to help just based on a screenshot, with a 3DM model file I'll have a much better chance of examining things and seeing what might be going on.

But one thing to watch out for is that if your surface has any "self intersections" in it where it might be bunched up awkwardly in such a way that it folds back over itself in a small area, that can get in the way of being able to intersect it.

- Michael