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.
Did you recently have a forced liquidation? I think they do not allow margin trading for the remainder of the day if that is the case. I'm not 100% sure, though.
The only thing close to that I've had is a 'RegT' quasi margin call warning me my daytraded position was over two times my account balance at the end of the day, and when I didn't reduce it they did it for me. I've let this happen a few times but the last time was a number of weeks ago.