I have a funny feeling the disk might be getting gummed up in the ablative. You don't notice while you're still in the soft ablative but it won't get through the barrier. Try getting off as much ablative as you can, put on a fresh disk and then work the barrier. And just buy disks by the 50 or 100 box. I'm a big fan of 3M sandpaper but not on the bottom, I just go through tons of Norton.