I can't tell exactly by looking at the picture, but if you have a sharp tool and you're not taking passes too deep, perhaps you're making the mistake I did when I failed to cut my first threads.
One word explains it: backlash.
Specifically, I don't have a thread dial, I would run one pass, then to reposition the tool I would reverse the lathe attempting to drive the bit backwards hoping it will take the same path it did when cutting material just in reverse. Backlash meant that the bit was slightly misaligned when returning resulting in my tool ruining the thread on the return pass. It looked like this picture too.
The correct thing to do is to withdraw, the cutter a known amount back before reversing.
Another backlash issue I had was that I drove that lathe back right to a point where the thread woukd start. Then I had no leading undercut so the tool would cut immediately before backlash from reversing worked it's way out through the gears. The first mm or so of the thread was a mess. The lesson? Always run few mm in direction of threading first before cutting. A 2mm undercut is all it takes next to a shoulder.
Finally, another issue that has nothing to do with backlash was that I was cutting too deep ruining the thread on the last pass. I'm now using the "no-trig" "measure thread depth with your cross slide" technique from Joe Pie video. And all my threads are
beautiful 
