This happens with many greats and Messi is one among them , Marco Van Bastan of Holland, Ruddi Gullit , Scifo etc are few to name here. There can be several reasons for not not performing/ contributing in big tournamnets either the opposition is more conscious/ mindful in making strategy or they are heavily marked. Let us hope he overcome this and do wonders for Argentina like great Maradona did.
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He might break the record of highest number of goals in a single season but as you told he is yet to fire big guns in bigger tournaments. Even in last FIFA world cup there was a big expectation from him but nothing great happened!