Quote from IB Salvatore:
If you leave the order in the simulated state (dark blue), our stock loan department begins to search for the shares. When they find the shares, the order will be submitted. You can cancel the order if waiting doesn't fit your strategy.
Quote from sam13:
Before this change, if the stock was not available then your order was simply cancelled and you had to keep resubmitting it to check if some stock had become available.