Question
I have 12 AVs, each a different variety. They have always bloomed prolifically, but about 6-8 weeks ago every one of them stopped blooming. I have always kept them near a north window with 150 watt bulbs on over them all day.  I used to water them about once a week, which kept them moist at all times, but recently started letting them dry out quite a bit before watering/fertilizing.

Answer
AVs must have sufficient light, water and fertilizer to bloom.  It looks like you have been doing very well until just lately.

Remember if these plants have been in bloom for a long period they may be taking a short rest.  Look for buds forming in among the leaves.

You might consider changing fertilizer.  I change each two months to get the higher numbers working.  For instance if I use 15-36-15 for two months, I might change back to 20-20-20, then go for another with different numbers.

Each fertilizer has different micronutrients and these should also be considered.

I suspect your plants are just taking a rest.  Allow them to do so.  Buy another plant for color and wait for your others to begin blooming.

Sounds like you have very good techniques for raising AVs.

Nancy