It's all of the above as we've said here, discussed here, been disgusted by, for the past few years.
Yes, play design, play calling, are to blame.
Yes, player talent at the wide out position.
Yes, Lamar's inability to throw consistently, accurately out side the numbers and down field.
It all plays together. Maybe because Lamar hasn't been able to make those throws in practice has lead to the play calling/ design.
Maybe the receivers can't run those routes properly/timely in practice, so Lamar doesn't throw them in a game when they are called.
Maybe, maybe, may be baby he will be true-u, maybe baby they will come through-u,
Maybe baby the passing game dreams will come true-u....