Sounds like the facet isn’t perfectly flat to the lap. Lower the head a tiny bit to increase the angle slightly.
It’s common to have to tweak things a bit while polishing. You can adjust the height to compensate for uneven polishing on the top or bottom of the facet, or use the cheater for uneven polishing on the left or right.
Knowing how to do this is especially important when polishing large tables.
I had this problem on a large amethyst emerald cut I did a few years ago. I tried everything to get that stupid thing polished. I spent hours over several days.
Like Mr. Balmer said, I thought my facet wasn’t flat. That wasn’t my problem though.
In the end I actually asked Bob Keller for advice. He suggested my prepolish was bad. And he was right, I had to go back a step to prepolish.
Turned out I couldn’t go from 1200 to polish like I usually do with quartz for that particular stone. I am not sure why it was more difficult than other quartzes. Try backing off to a 3k or even 8k, then go back to polish. See if that helps if what Alan suggested doesn’t work.