Of course it's not simple. That's why sporting directors at top clubs are paid in millions, given incredible support systems in terms of staff, and have access to vast scouting data. It's not supposed to be simple, thus all the plaudits when a club like ours get it right for such a sustained period of time. Moves involve the availability of a player, the cost both in transfer fees and salary, the desire of the player to join, other positional priorities, the manager's opinion, and on and on.
But don't for a moment think there aren't at least a dozen players at every position on the pitch who could help a City, a Real Madrid, or a Bayern improve. You may not know who those players are but you can be sure Pep, Txiki and his staff do...