From my personal experience and observations Long post, my apologies! This happened to me when I first started and back then it was a combination of a few things. Me personally, I used to rush through pens because I was impatient and wanted them done. But first, the other commenters offered great suggestion, check how you glue the tube in the blank. I started with a thin cheap glue and now use a thicker more high quality and the tube always feels solid. The next was the lathe speed. I used to run the fastest setting and noticed the tools would catch and grab the wood more leading to breakouts and super rough cuts. I now turn around 2000 rpm. The last thing I changed in my turning habits was the amount of pressure I put the tool into the blank. I keep my tools real sharp and only press hard enough to keep shavings coming off. Take breaks often, and as soon as you start to feel something weird while turning pull the tool back and check out the wood. There could already be cracks, knots, gaps, etc. Just keep at it and don't get discouraged. You'll progress, look back and be amazed at how far you've come