The system seems to "make room" for new types by concentrating new slots (with increased production) around the release times of the new models. You may actually find some of your orders have been pushed back - by as much as a year
to make this room. I had this problem with A330/A340 and it CAN be solved if you're careful:
Try to place some orders for a type in the same subfleet which you do not currently have on order (in this case A321). You will PROBABLY be offered 6 or 7 of the empty slots BEFORE your other deliveries but be sure this is true before proceeding. Once these are ordered, immediately change them from A321 to A320 (or whatever you need). There will be a small financial penalty but this will be easily made up for by having the extra 6 or 7 aircraft years earlier.
I did this with my A330 and it worked fine.
A bit of a bug really and very annoying. For example, in Jet Age, with the DC8, there are so many new models all the time that some of my orders were delivered NINE YEARS LATER than scheduled when I originally ordered them, so count yourself lucky this is just a few months.