Lots of reasons, joint accounts, name mismatch, weird names, fat finger by bank employee, missing odd info deemed necessary by the receiving bank, fraud/money laundering holds. One of my very normal wire transfers of not even a huge amount arrived next day for no reason, defeating its purpose, and neither Fidelity nor the receiving bank had any explanation, or really knew anything about it, without a formal trace. Fidelity after all is not a bank. They'd have to check with their own bank.