Seems to me it wil be a gradual effect, I mean basicalyit is a sponge drying out.
Also I think when you have vaped a bit the sponge near the heating element will dry out so you wont get much vapour however when you put it down for a bit it will get moister again.
Vapor production is the biggest clue.
jlcanary has some insight here. I have noticed that some liquids, and using a cartridge too long will cause the liquid to coagulate on the top of the cartridge filler material.
Try this when filling, if the cart is not brand new and needs to be filled. Take tweesers, pull out the filler, put on a paper towel, put liquid drops to fill the the reservoir half way. Replace the filler/cart material up side down (opposite to the way it was). If there is glue/silicone (some manufactures use) pick/pull it off first. Then put a few more drops on top (if needed) untill full. Does it not
vape like new???? This means we need a new cart, or to wash/refill the material/fiber filler.
Add: Another clue to this coagulation I have seen, is that you are only able to put 2 drops on top with no absorbtion, even though you know it must need more as you have been using it quite a bit.
