In order to get the premium, you have to wait for the price of the option to decay. It doesn't appear right away. As the value of the option you sold decreases, its value in your portfolio will increase because it's a short position.
The problem with selling ITM calls is that the buyer can force you to provide the shares at the ITM strike price. That ITM strike price is likely less than you purchased the shares for. So you're selling the shares for LESS than you bought them for. Which is a loss.
So the problem here is that if the option gets exercised, you lose stupid amounts of money.