Only happened to me when I moved a large-ish amount of money out of an internet savings account to an online broker where I hold my share ISA. This was to fund an ordinary dealing account and I used the brokers system to load up the share account from my current account, but there is a limit of £25K per transaction. The 2nd tranche was blocked and I had to phone the banks fraud department and confirm that I still had the card and that that destination for sending money was valid.