Shared Fitness Workouts on Our Apple Watches

My son and I share our fitness workouts on our Apple Watches. I was able to reply to notifications when my son finished his workouts but now I can’t. When I reply to his workout notifications, I immediately get a “failure to send message” reply. I’ve updated all software on my iPhone and Apple Watch. Also my son is able to reply to workout notifications without a problem. What can I do to correct this?