"I have also noticed a pattern, i.e. often stocks are not available to short when you would like to do so, then they BECOME available later." I have noticed this also. It seems to be more difficult to find shorts when shorting is what many traders want to do. I wonder if, when traders move significantly to the short side en masse, IB makes the opening of further short positions difficult or impossible as part of a risk minimization strategy for the company. On long side trades they have the stock as security for a violent change in the market, but on the short side, they are bound to replace the borrowed stock and are entirely dependent on the traders to ante up real money to meet margin calls in the event of a runaway to the upside. OTOH it could just be that when shorting is very popular the inventory of stock available to short gets depleted very quickly.