Many brands of black thread can cause problems because it's the most heavily dyed. I have used black Presencia 60wt cotton with success, but other brands have caused problems... It can be a couple of different things: the dying can cause the thread to be a bit weaker and/or I've found that the thread can be a bit stiffer and easily jump out of the take up lever on the top and wind around the spindle in the bobbin case (back lash). You can try bobbin genies to see if it eliminates the back lash issue. And finally, your statement "...why bottom line has fallen from grace with my machine..." sounds like you've successfully used bottom line before. If this is a new purchase, I'd contact the seller. You may have just gotten a bad batch.