Zyra plants not giving assists

During a dragon fight, my plants were in the bush between Dragon pit and Red buff, I had two ranged plants up and they were hitting the enemy Jax. He walked out of their range, but my jungler caught the kill. When Jax died, I didn't get any assist for it. I seemed to only get assists when a Q or E or R or AA hit them, but not when they took damage from (just) my plants.