We have no way of knowing, you have to ask the bank that blocked the deposit.
Some possible reasons are,
•you used the wrong account number or wrong routing number
•you are not the owner of the bank account
•it is a joint account and the deposit was in one person's name
•it is a joint deposit but the account is in one person's name
•you were using the refund processing service, and your bank did not recognize the bank that was sending the money because it was not coming directly from the IRS
