There are several options here:
1. you have been outbid by another manager.
2. your bid was lower than the market value of the player at the time of the transfer. The further your bid is below the market value, the lower the probability that the bid will be accepted. You should check your bid again before the transfer date, because the market value of the player may have changed since the bid was placed and your bid may have slipped below the market value.
3. a player transfer takes on average 1 hour, but in exceptional cases it can take longer. You should take this into account for your transfers so that you do not run into difficulties shortly before the match day kick-off.