.This has been explained in several other posts. Basically the way the system works, is that when any airline makes their first order of an aircraft type, it takes the first slot and then one about every two weeks or so. Then if you order more of that same type, the new order gets slotted behind the first order, even if there are open slots before available.
This is to prevent an airline from hogging or blocking a type of aircraft from competitors. Might not be Real World, but it helps make the game fairer for all of us.
These forums have a wealth of information in them. Take some time to look at older posts before asking questions, it will be faster for you