It's strange that the humidity would suddenly drop like that. Will the Boveda packs have dried out at the point when the humidity drops, or do they still have the gel-like feeling when you squeeze them? When I first moved my cakes to crocks, they were fairly dry and it took a couple of weeks for their internal humidity to increase. At this time the measured relative humidity in the crocks was lower, and the packs would dry up faster. Once the cakes had "re-humidified" (to the Boveda pack level) the relative humidity in the crocks increased, and the packs now last significantly longer than they used to.
My largest crock is 15 liters and holds 15-20 cakes. I use two 60 gram packs, and the only seal is a dish towel and a dinner plate (on top of which we keep bags of rice).
For smaller crocks, I only use one pack.