I knew this is a pretty old issue, but I had encountered this issue as well, after half an hour spent with support online chat, finially got it resovled.
Basically have to go through these steps:
Not ideal but glad my old vivoactive HR watch is still functioning well.
Thank you for posting this process, it seems to have fixed my watch not picking up my miles during my last two runs. Amazing how addicted I am to this watch, and how it affected my last 2 runs without the GPS picking up my distance! Thanks again.
Garmin 745 was stopping recording distance mid way through activities. Tried the on /off/ reset. Tried firmware update in GRMIN express . Tried GPS 'Saturation' (I.e. Waiting for a while before setting off). It was GPS+ Glonass issue. Switched to just GPS for runs and works fine.
Had same issue for 'walks'' and realised you need to change GPS setting for each activity.