Before for day trades I used to be able to buy stock up to my 'buying power' which is the normal four times the account balance. If I chose to keep a position overnight I was limited to two times my balance. Now (today and last week) I'm getting a message when I try to short a stock saying my 'initial margin' is limited to my account balance, in other words I can only sell short up to what I have in my account rather than its buying power. I've only been doing much large position shorting for a week or so, but is this a new restriction or has it been around for a while, because I know I've had much larger long positions without any problem at all? Also, IB has done a few updates to their TWS software in the last couple of months which resulted in at least one bug in the real time chart display (which I reported to them and now is supposedly fixed) so I don't if that's a part of this or not.