I have had this problem also. I uploaded over 2,000 songs though, doing also going from iTunes to Google Music.
It looks like all of my songs transferred over. If I'm using my desktop computer to check the website music.google.com for my library and playlists, it's all there. When I go to use the app on my phone, the music is all there, but the playlists show up as empty, or have only a small number of the songs it's supposed to have. Frustrating, because most of the time I want to pick a playlist to listen to.