I agree with Gestalt, I think the striker you currently have may not fit with your formation.
If you play 4-3-3 like Barca or Chelsea, you need a striker who can create chance for team mate as well as making something out of nothing for himself like Messi, Augero, or at least a strong target man who can hold the ball under pressure to bring team mate in like Drogba, Ibra.
If you play 4-4-2 or 4-2-3-1, you may need a pure poacher like Man U hernandez, he will need his teammate (the wingers, second striker, attacking mid... ) doing all the hard work to give him the ball in good position. His only job is being in the right place at the right time and score.
Other reasons could be your striker is not in form ? not fully adapt to the new team ? language barrier ? feeling jade or stressful ?
Anyway, the advice is buy the most suitable, don't buy the best possible (unless it's someone who are capable of magic like Messi).