I don't have that problem unless I singe a carto by letting the stuffing get too dry. I keep cartos damp always and can run about 20ml through with good flavor.
Some other notes (with 1ml popular stuffing-type cartos)
- I use THIN liquids around 80PG/20VG or less, and a couple of V4L WOW liquids
- I use CLEAR light color liquids that do not drop flavoring sediment in the stuffing. I avoid all dark, opaque or cloudy liquids or any other liquid that drops flavoring sediment. Flavoring sediment will crap up a carto real fast, ruining the drag, the airflow, the flavor
- I do not use the condom method. An overfilled carto has little flavor. I drip into the stuffing (with carto off battery) and I clean it up when done. I try to keep the carto nicely damp but not soaked. I use a plastic or delrin drip tip as a mouthpiece so it's easier for me to remove it for checking the stuffing or adding juice.
- There is nothing you can do about coils getting baked on juice coating and losing flavor other than using the clearest juices you can. No kind of cleaning will get the baked on coating off the coil. Cartos have limited lives.
If you are getting less flavor with band new prefilled cartos - has your battery gotten weaker? Sometimes if just a small amount of juice gets in the battery it can degrade it without killing it. I do recommend manual batteries and I recommend doing your best to make sure your cartos don't leak onto the end of the battery. Also always wipe off the battery threads before putting a battery on a charger or a film of juice residue will build up on battery and charger and cause bad charging.
In addition, everyone goes through a period where they loose taste. You can try to avoid that by remember to to smell the vapor, making sure your sinuses are clear, and not vaping the same flavor over and over and over. I use 2 contrasting flavors and alternate vaping them - I actually carry 2 models, one with each flavor. And remember to drink plenty of fluids. Hot fluids and alcohol can dull your sense of taste/smell, cold water can help refresh it.