What type of material is the phone case? It appears to be plastic. Am I assuming correctly?
For plastic cases, 275°F for 300-350 secs is recommended. Maybe, you just lifted the press too soon.
Also, did you do the initial 5-minute warmup for the machine’s fixture and the 2–10 second preheat cycle for the sublimation film during the press itself?
Did you do a hot peel or cold peel? Hot peel is recommended.
It looks really good for a first try, though! 
Edit: I think you should share this phone case in this post: 
Share Your H17 Projects and Win Bonus Film Supplies