The same thing happened to me today. I also had a Pebble Steel and wanted to add it to my iPhone 6.
The Pebble app on the iPhone 6 would not even let me sync the two via bluetooth. I would hit the pairing check mark on the Pebble Steel, but there was not response.
I finally went away for a few hours, and when I came back, I tried it again. This time it worked. I was able to sync via bluetooth and download the latest firmware.
I think, this time, there was a glitch with the Pebble Servers or something. This is because I had another problem at the same time on my Pebble Classic and Pebble Time. I could not longer more than the Tic Toc watch face, and the kickstarter watch face. Nothing else was available literally!
When I came back a few hours later, I had to reset both to factory reset, and re-sync and install the firmware. Then everything was fine again, including all the watch faces that had disappeared.
However, I think this is all a taste of what will happen once the Pebble Servers go away. No way to download firmware, after factory reset. No access to watch faces other than Tic Toc. No apps other than Calendar, because that was the only app left on my Pebble watches during the glitch.
Glad everything seems to be back to normal for now.