Allow for any music in the cloud to first be downloaded to your iPhone, before it then transfers to the watch....
"place the watch on the charger to complete the sync"
I had trouble, too. My problem turned out to be that I was trying to sync a playlist larger than 1 gig. A gig is the most music you can hold in an Apple Watch. Also, it's not easy to find out how much data is contained in your playlist, but start by considering that.
So, you might want to try to sync a shorter playlist which you know is less than a gig. If that works, that was your problem.
The simplest of tasks make my life so difficult. But thank you so much. this helped a lot.Connect both you Watch and your Phone to their chargers
The apple watch app is buggy. Kill the app forcibly and open it again and you will see progress. I had to do this a few times to get it all working.
Thank you, that was what worked for me, I had the iwatch on 50% and even more but didn't started to synchronise until you connect it to powerConnect both you Watch and your Phone to their chargers