Hi Jesper, yes like Brian says it's likely to be the skimming overlap with these already cut neighboring areas that makes it difficult for the difference to be calculated:



If you make those cutting objects to not overlap the other adjacent areas it should reduce the complexity of the boolean calculation, so position things more like this:




Or also you can use trimming like Brian mentions above - sometimes it can work well to extract some portion of the object using Edit > Separate and then you can work on trimming just that one piece all by itself and then rejoin it to the main piece when you're done.

I've attached here a zipped 3DM file with the bottom all level now, I also did some tuning up on the bottom face of the outer ring - I made it into one single big trimmed face, it was previously a little messy with some separate pieces with a few little slivery bits in some areas.

- Michael