When you short shares you are borrowing shares from someone who has a position in the company. The original owner of the penny stock, may ask for their shares back at anytime. This is can cause what is known as a ”buy in” and can usually happen after a few days or 14 days. A broker may issue a ”buy in” if you are in a losing position on one of your short sells. You essentially have little or no control of what happens, causing you to lose on the trade as the broker will buy back the shares at current market price. “Buy ins” cause short squeezes to increase the share price artificially as traders are being forced to cover there shares at around the same price levels.