So there has been a lot of issues with fans involving the stopping power of the bowcaster vs Kylo Ren. Whereas it was shown to knock back stormtroopers in one hit, and effectively have one-shot kills, it merely made Kylo bleed a lot, and wound him.
I believe, however, that there is a justifiable explanation besides plot armor: namely, his ability to stop blaster bolts.
In the first part of the film, he is shown to be able to stop Poe's blaster bolt in mid-air. He later does this again, during the raid on Maz Kanata's castle. I believe that it is possible, and believable, that he could have attempted to do this to Chewbacca's bowcaster bolt inside the Starkiller base, but due to his complicated emotional state, mistimed it, and merely slowed it down to a point where it didn't cause a lethal sound.
That's my explanation. What do you think?