This was a recurring problem when I was with CyberTrader, and I'm talking very liquid stocks like NVDA. I bugged them constantly, was always told that things would be better soon. Finally late last fall they worked an arrangement to borrow shares from Schwab (gee, Schwab had owned them for a year and a half and they finally got around to it...), also said they would get on Penson (their clearing house) to have more shorting inventory available. I think things have improved there, don't know for sure as I gave up on them for that and other reasons.