Transferring large amounts

[jtaylor69 (Jordan Taylor)]

Wasn’t HSBC by any chance was it?
If so, this may be why:

Payment via Bank Transfer from HSBC

Unfortunately Faster Payments to a Monzo account (or to any bank account) from HSBC accounts need to be sent before 14:30 to arrive the same day. If they are sent after that cutoff time, there’s a chance that they may hang until the following morning