Codes/discounts aren't available across the board. For each room type in each resort, Disney has a certain secret occupancy rate in mind. Once that occupancy rate is reached, a code is not longer available for that room type at that resort for those travel dates. If an occupancy rate is already achieved for those dates on the first day the code is available, then that code can't be used for that room type for those dates, unless there are sufficient cancellations that subsequently bring the actual occupancy rate below the target occupancy rate.
Let's look at an example.
CBR has 2,112 rooms, 384 of which are "preferred location" (there are other types of "upgrade" rooms as well, but I don't know how many, so let's just ignore that for this example!

).
Disney doesn't want to put rooms "on sale" (discounted price) if they are selling very well. So let's say they set the target occupancy rate for preferred location rooms at 80% ... meaning that the code/discount won't be available for a preferred location room for any given date in which 80% or more of those rooms are already reserved. 80% of 384 is 307.2.
So let's say that a code was released TODAY for a few months from now. Let's say that as of today, only 298 of those rooms (for your dates) are rented ... that means you can get the code!!! But if by tomorrow 10 other rooms get "sold," that will bring the total occupied rooms for your dates to 308, which is over 80%. That means that if anyone else calls up trying to use the code for a preferred location room at CBR for those same dates, they will be told that they can't use the discount code for one of those rooms.
Whether or not you already have a reservation makes no difference. You can't just automatically get the code applied; if the occupancy rate (including YOUR existing reservation) is over the target occupancy rate, then that code just simply can't be used.
So whether you already have an existing reservation or you don't have a reservation yet, you have no advantage or disadvatage in getting a discount code applied.
And just because you're told that you can't apply a code, don't be discouraged! Keep calling every day, and there might be enough cancellations to make a difference. It worked for me! I was willing to call every day until the cows came home, but it took only a week!!!