That's easy he's been crap. His 1st touch is awful, he's slow and very cumbersome. If you can't score in La Liga where u get all the time in the world then you've no hope in hell of doing well in the EPL.
I just think it's strange, very strange, in the summer they were talking big money for him, he goes to AM 6 months later and his fallen so far they want to loan him. I can't believe costa will be going back to AM so they are one In one out but Gus did say something in his press conference which could have insinuated a lot of things.
He's not shit but to be successful in the top leagues (England, Germany, Spain) you need to be 1 of 2 types of striker.
1) in the Messi/Ronaldo mould and have the quality, confidence, and quick thinking to be able to beat your man, then beat him again before scoring. Granted these players are very rare.
2) you need to be instinctive, create half a yard, aim for the corner and shoot. Players like Kane, Benzema, Costa, Lewandowski, Lukaku, Shearer etc etc etc
Players who hesitate and there's tonnes of them will never become consistent performers in the top divisions. Scoring 30, 40 a season in Mickey Mouse leagues like Holland, Portugal, France, Belgium where there's only 1, 2 at the max decent sides is no great achievement when you fail miserably in the top 3/4 leagues in Europe.