It could be a number of issues - hotspots, poor wicking, too strong juice, poor airflow... all of those will cause harshness.
Easiest one to check is making sure your airflow hole is directly in front of the coil - even as little as 1/4" off can make a difference to the harshness of the vape.
If that doesn't solve it, check for hotspots where the coil isn't making contact with the wick down the length of it. A hotspot can cause a pretty harsh vape all by itself.
You said stock coil - what do you mean by stock coil? You might consider just building another coil - see if the harshness remains. If so, try a different juice to eliminate that as a possibility.