I'm having the same issues. I've contacted Apple support, restored phone, contacted fitbit support, deleted app and reinstalled it, removed the fitbit from the app and added it again, restarted phone, checked for ios and fitbit updates, I've reset the fitbit and all of this to no avail. The good news is that I'm able to successfully sync my fitbit alta with my PC which means it's not an issue with my device but an issue with my iPhone 7 ios 11.2.6. I'm hoping it corrects itself and soon. Apple support can't help and fitbit support can't help either. One more thing, a fitbit user recommended that I let the fitbit battery drain completely then charge it up again. Supposedly this fixed his problem so I'm hoping this works for me too...fingers crossed here.