OK, I have had this problem on bowls and now it is showing up in my spindle work. Side note: I started turning years ago and NEVER had this problem. Now I get a good lathe and good tools, start back and nothing but problems! End of Rant. I find I use a wood mallet all the time. I have a couple and decided to make another. I turned the head from a pecan square, the handle was hickory.
Look at the head and you can see rings around the head. These rings are low spots that you can feel when you run your hands across it. They don't run all the way around the part. It's almost like an ellipse instead of a round cut.
There are no knots (hard spots) that the tool would be bouncing on. Of course they don't show till you start to sand the part and these spots being low don't get sanded.
OK, now what the heck is causing this?????