This is what I got from IB:
Dear Trader,
Thank you for your email. It is not a fixed rule of thumb, but you may want
to wait until the trades settle. Meaning 3 business days. Then in order
for you to short the stock it must be marginable. Usually this is not available
for 30 days, but this is not fixed either. The best advice I can give you is
foryou to keep trying to short the stock until you can.
John W
IB Customer Service