What phrasing are you using? Personally I find it is most effective saying:
“play ____ from emby”
“play artist ____ from emby”
etc
But knowing what terms you were using would be useful for the Skill author to consider additions to their vocabulary
Would also be interesting to see the Skill logs from /var/log/mycroft/skills.log to see what your requests are being transcribed as by the STT, and then how the system is handling them. It may be the Emby Skill, or it could be the Common Play Framework doing something.