I came across this same problem and I think I have it figured out. I also closed some old accounts and hid them. The transactions in the hidden accounts were not showing in the transaction history. I went to the report for that security and clicked on customize, under accounts I clicked to show hidden accounts and selected all. The transaction history now showed the transactions from the closed/hidden accounts which explains why I could not delete the security. I you still want to delete the security you can look up delete security in the help menu. It tells you to delete each transaction listed on the report before you can delete the account. This of course will delete the activity in the closed accounts and could mess things up if you ever want to look them up.