Similar issue. First, to those having trouble with getting Alexa to play Spotify properly... Alexa will try and get you to buy Amazon Prime to play music but there's an option within the app to make Spotify the default music player when sending voice commands. Do this first.
Now, here's my issue. I can get Alexa to play music and playlists from Spotify just fine. After you make Spotify the default, you don't have to say "play so and so from Spotify", she's use whatever service is the default. Good thing. If I just say "Alexa, shuffle my music", she plays the same four artists and pretty much the same songs each and every time! No luck shuffling playlists either. I have playlists in Spotify with hundreds of songs with the sole purpose that I won't have to hear the same song twice. So I go "Alexa, shuffle (playlist name)". She'll play the first song in the playlist. So I go "Alexa, skip 25 songs". She skips some songs but pretty sure not the number specified. If I just go "Alexa, skip", she goes through the handful of artists she always plays.
I'm beginning to think it's some Amazon scheme to get people frustrated and into buying Amazon prime. Not atypical for this kind of device/service (works wonders for Apple stuff). I got the Echo Dot as a gift at work so not complaining, I enjoy the to-do's, alarms and other features. But I'm an audiophile, music is super important in my everyday life. Alexa fails in that department.
Alexa also has the bad habit of playing two songs from Spotify at the same time! When I get fed up with the above issue, I switch to playing from my iPhone through Spotify Connect. So I go "Alexa, stop" (to stop her playback). Then I play something from the phone and a few seconds later she resumes playing whatever she thinks I want to hear, on top of my song!!! so I need to go "Alexa, stop!" once again.
This voice commend thing can be a convoluted mess I tell ya!