Hi Tom and welcome to ECF!
I've never heard of FIN cartos either, so I googled them. Ouch.... Are these the cartos you're using?
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If so, please use up what you have and don't buy them again.

16 bucks for a 5-pack is highway robbery--for starters. Also, they list peppermint oil as one of the ingredients in their menthol cartos--oils are a big no-no in
vaping. So they (the seller/manufacturer) are not only ripoff artists, they also have no idea what they are doing. Just don't buy them anymore, please.
You can refill them a few times, depends on the quality of the cartos and the
eliquid you're using; dark, sweet juices will gunk up your coils in no time while the clear and clean ones will help your cartos last longer. Roughly, a carto can process 5-8 ml of eliquid, more or less, before it needs to be thrown out. They are disposable. BTW, the filler is polyester, not cotton.
I suggest you buy some good blank cartomizers--LR Boge or LR Smoktech single coils are my favorite cartos--and some quality eliquid and fill them yourself. Liqua and Mt Baker Vapor make good and inexpensive juices. Visit our Eliquid subforum to find more reputable juice vendors.
510 BLACK BOGE LOW RESISTANCE Blank Cartomizers
or you could try a clearomizer--no filler--this is a good one:
Kanger 510 T4 Clearomizer
I like Liqua's Cappucino--I mix it with their Coffee to make it less sweet. Stay away from the Grape, though. It's nasty.
LIQUA premium Italian flavored electronic cigarette liquids.
Good luck and happy vaping!