1) Bids made outside the transfer market (e.g. through Scouting Missions) are more likely to be refused by other managers because, generally, they may hold tactical or sentimental value to their existing club.
2) Another club may have signed them first. Your competition never sleeps (no, we don't know how they do it either!). Whoever signs a player first wins the race. All other bids get refused.
3) If a manager decides to offer a player for loan, all current negotiations with him will be cancelled.

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