Maybe it was because it was the end of the season, but I started playing ranked and got up to 15 pretty smoothly with my priest deck (season ended, I don't know how far I could've gone). I changed the deck afterwards and i'm having more of a difficult time now but I'm not sure if that's because all the ranks reset or if my deck's a bit more shit now.
the end of the season is easier to rank up because the vast majority of good players have already made it to the high ranks.
the beginning of the season is always difficult because those people are now at the same rank as you, so naturally you are going to lose more.
and on top of that you're playing the hardest class in the game.