How much is "quite a bit" of ventilation? (Everyone's got a different idea of what is a 'normal' amount, and sometimes what seems like quite a bit, isn't really).
OTOH if you really HAVE vast quantities of ventilation, is it possible that you've gotten rain or snow come in to dampen the litter? Which will cause more ammonia to come off.
You might try *not* stirring the litter, but instead just adding fresh stuff on top. Another thing to try (instead or in addition) is to remove the under-the-roost deposits of poo, which are usually the worst offenders yet very easy to remove in one fell swoop.
If none of that helps, you can try something to control ammonia (lime, Stable Boy, Stall-Dry, whatever), but it is possible you simply may need to experiment with different ways of managing the littter pack to find what works in your particular situation. There is no "the" deep litter method, there is a whole constellation of different ways of doing things in that general manner, so rather than trying to follow a particular cookbook procedure it is usually necessary to experiment and adapt until you find something that suits your circumstances.
Good luck, have fun,
Pat