Thanks Scorch, with the walnootolie sentence, that did it! I had it set to 0.5 mm.
I also tried to do my halftone with it set to 0.0001 up to 0.01 but that gave calculation times of 300 minutes. After stopping the calculations and writing to axis it did seem to show many more of the smaller points so it does seem that this is the cause of my missing dots.
Is this accuracy number something that need just to be played with? And is it really a dimension or just a number?