With my cats, usually it only takes a couple shots before everything returns back to normal. Usually 1 shot 2-3 hours late will not have much affect. Are you hometesting? You will be able to tell when you test for the next shop. While the recommended shooting times are twelve hours apart, if you are off of that schedule by 1 or 2 hours that will not make a difference. You just want to avoid giving a shot less than 10 hours after the previous shot, especially if you are not hometesting.