OK, this is what I remember from going through this last year:
There are three types of nominations:
1. Principle nomination (MOC says I want John Smith, and here are nine alternates if John Smith isn't qualified. Most MOC's do not nominate this way. If your son or daughter has a principal nom and the USNA says they are qualified, then an appointment will be offered.
2. Nomination with a ranked slate. (MOC says here are names listed one through ten in order of preference. USNA starts at the top, and the first on the list that they deem qualified gets offered an appointment.) Most MOCs do not nominate this way.
3. Nomination with a competitive slate. (MOC says here are ten names in no particular order, and USNA picks the one they want and offers an appointment.) This is how most MOC's nominate.
If your DS/DD doesn't get offered the appointment from the MOC's slate, then their name goes into the pool. Once all the MOCs appointments are offered, then the academy fills out the rest of the class from the pool. Appointments from the pool are based on the candidates WCS (not sure the exact translation...whole candidate score, or weighted candidate score I believe.)
Appointments from the pool can drag out. After a certain point, they will send out notification to a small number that they are on the waiting list, and they will sent TWEs to the rest. (Our wait list letter came the third week of April last year.)
Just remember, no news means at least you still have a chance!
How do you know if you're in the pool? If your MOCs slots have been filled and your DS/DD did not receive the appointment and they are 3 Qd, then they are in the pool.
All MOCs are given the opportunity to call the appointee, but not all do.
The bottom line is that until you get the TWE, you still have a shot. If you do get a TWE, be sure to open it. The wait list letter comes in a TWE and you only have a limited number of days to respond if you want to remain on the wait list (I think it was 5 or seven days. We lived in an area that had very slow mail delivery, and by the time we received the letter, my son only had one day to answer! So don't postpone opening a TWE! Be forewarned VERY FEW go on the wait list, and VERY FEW get appointed from it.)
I'm sure others will have more detailed answers for you...I just remember having all your questions and NEEDING answers!
I hope that helped.
Best of luck to you!