When you book doesn't seem to matter for requests. The resort doesn't want you to call them and the phones are answered at a call center.
Several factors determine request success rate, resort, size of room, check in time, what you requested and just the luck of someone matching your request checking out when you are checking in.
With over 40 WDW stays we get our request filled about 50% of the time.
You should call or e-mail member services. DVC has asked members not to contact the resort directly. If you have general requests (high floor, low floor) you can also do online check-in.