There are several reasons why an order does not get processed instantly.

All orders are queued by the time the order was placed and the order price.
The earlier an order was placed the higher in the queue the order will be placed. The oldest orders at a certain price will be executed first.

So if there are already orders at a certain price it may take some time before your order gets executed at the same price.
You can either leave the open order in place and wait or cancel the order and change your order price.