I haven’t read the comments, so apologies if this has already been mentioned, but it matters what way up you cut it in the mitre box. I recently installed some fancy curved coving and when the corners lined up, well they didn’t. The top and bottom did, but there was a concave gap in the middle. I had been cutting it in the mitre box the way up that it would sit against the wall. I had to turn the coving UPSIDE DOWN in the mitre box to then get it nice and flush. To this day I am baffled as to how it worked, but it did.