A very common problem, many possible issues. May be due to the mandrel, bushings, several others. (But practice with each - simply turning practice blanks with bushings; blanks with glued in tubes/no bushings; and with TBC; and checking with calipers will help. Also insure your mandrel rods are plumb).Not sure how, but the pen tip is perfect on one side, but the wood is proud on the other. Also uneven fit at the back half against the center ring. I think maybe the blank wasnt square?
At any rate, harder than it looks on YouTube!